% --------------------------------------------------------------------------
% the CHEMFORMULA package
%
%   typeset chemical compounds and reactions
%
% --------------------------------------------------------------------------
% Clemens Niederberger
% --------------------------------------------------------------------------
% https://github.com/cgnieder/chemformula/
% contact@mychemistry.eu
% --------------------------------------------------------------------------
% If you have any ideas, questions, suggestions or bugs to report, please
% feel free to contact me.
% --------------------------------------------------------------------------
% Copyright 2011--2020 Clemens Niederberger
%
% This work may be distributed and/or modified under the
% conditions of the LaTeX Project Public License, either version 1.3
% of this license or (at your option) any later version.
% The latest version of this license is in
%   http://www.latex-project.org/lppl.txt
% and version 1.3 or later is part of all distributions of LaTeX
% version 2005/12/01 or later.
%
% This work has the LPPL maintenance status `maintained'.
%
% The Current Maintainer of this work is Clemens Niederberger.
% --------------------------------------------------------------------------
\RequirePackage{expl3,xparse,l3keys2e,tikz,amsmath,xfrac,nicefrac,scrlfile}
\usetikzlibrary{arrows.meta}

\ExplSyntaxOn
\tl_const:Nn \c_chemformula_date_tl {2020/02/01}
\tl_const:Nn \c_chemformula_version_number_tl {4.15}
\tl_const:Nn \c_chemformula_version_suffix_tl {i}
\tl_const:Nx \c_chemformula_version_tl
  { \c_chemformula_version_number_tl \c_chemformula_version_suffix_tl }
\tl_const:Nn \c_chemformula_info_tl
  {typeset~ chemical~ compounds~ and~ reactions}

\ProvidesExplPackage
  {chemformula}
  {\c_chemformula_date_tl}
  {\c_chemformula_version_tl}
  {\c_chemformula_info_tl \c_space_tl (CN)}

% --------------------------------------------------------------------------
% warning and error messages:
\msg_new:nnn {chemformula} {missing-argument}
  {
    Maybe~you've~forgotten~to~add~the~second~argument~to~name~syntax?~
    "!(<name>)(<chemformula>)".\\
    Or~you~have~forgotten~to~add~a~group~between~an~exclamation~mark~and~
    the~following~parenthesis?~"!~{}()"\\
    Either~way~-~I~don't~know~what~to~do~ \msg_line_context: .
  }

\msg_new:nnn {chemformula} {new-arrow}
  {
    You've~tried~to~define~the~arrow~type~\tl_to_str:n {#1} \c_space_tl with
    ~\token_to_str:N \NewChemArrow \c_space_tl but~it~already~exists.
    ~Choose~another~name.
  }

\msg_new:nnn {chemformula} {renew-arrow}
  {
    You've~tried~to~renew~the~arrow~type~\tl_to_str:n {#1} ~but~it~doesn't~
    exist.
  }

\msg_new:nnn {chemformula} {unknown-arrow}
  {
    The~ arrow~ type~ \tl_to_str:n {#1} ~ doesn't~ exist~ \msg_line_context: . 
  }

\msg_new:nnn {chemformula} {new-bond}
  {
    You've~tried~to~define~the~bond~type~\tl_to_str:n {#1} \c_space_tl with
    ~\token_to_str:N \NewChemBond \c_space_tl but~it~already~exists.
    ~Choose~another~name.
  }

\msg_new:nnn {chemformula} {renew-bond}
  {
    You've~tried~to~renew~the~bond~type~\tl_to_str:n {#1} ~but~it~doesn't~
    exist.
  }

\msg_new:nnn {chemformula} {new-compound-property}
  {
    You've~ tried~ to~ define~ the~ compound~ property~ `#1'~ with~
    \token_to_str:N \NewChemCompoundProperty \c_space_tl but~ a~ property~
    `#1'~ already~ exists.~ Choose~ another~ symbol.
  }

\msg_new:nnn {chemformula} {renew-compound-property}
  {
    You've~ tried~ to~ redefine~ the~ compound~ property~ `#1'~ with~
    \token_to_str:N \RenewChemCompoundProperty \c_space_tl but~ a~ property~
    `#1'~ doesn't~ seem~ to~ exist.
  }
  
\msg_new:nnn {chemformula} {new-addition-symbol}
  {
    You've~ tried~ to~ define~ the~ addition~ symbol~ `#1'~ (input~ symbol:~
    `#2')~ with~ \token_to_str:N \NewChemAdditionSymbol \c_space_tl but~ a~
    symbol~ with~ the~ name~ `#1'~ or~ the~ input~ `#2'~ already~ exists.~
    Choose~ another~ name/input.
  }

\msg_new:nnn {chemformula} {renew-addition-symbol}
  {
    You've~ tried~ to~ redefine~ the~ addition~ symbol~ `#1'~ (input~ symbol:~
    `#2')~ but~ a~ symbol~ with~ the~ name~ `#1'~ or~ input~ `#2' doesn't~
    exist.
  }

\msg_new:nnn {chemformula} {new-symbol}
  {
    You've~ tried~ to~ define~ the~ symbol~ `#1'~ with~ \token_to_str:N
    \NewChemSymbol \c_space_tl but~ a~ symbol~ `#1'~ already~ exists.~
    Choose~ another~ input.
  }

\msg_new:nnn {chemformula} {renew-symbol}
  {
    You've~ tried~ to~ redefine~ the~ symbol~ `#1'~ with~ \token_to_str:N
    \NewChemSymbol \c_space_tl but~ a~ symbol~ `#1'~ doesn't~ exist.
  }

\msg_new:nnn {chemformula} {command-deprecated}
  {
    The~command~\token_to_str:N #1 \c_space_tl is~deprecated.~Use~
    \token_to_str:N #2 \c_space_tl instead.
  }

\cs_new_protected:Npn \chemformula_define_deprecated:NNn #1#2#3
  {
    \cs_new_protected:Npn #1
      {
        \msg_warning:nnnn {chemformula} {command-deprecated}
          {#1} {#2}
        #3
      }
  }

% --------------------------------------------------------------------------
% variants of kernel functions:
\cs_generate_variant:Nn \keys_set:nn         {nV}
\cs_generate_variant:Nn \tl_if_blank:nF      {V}
\cs_generate_variant:Nn \tl_if_in:nnT        {x,nV}
\cs_generate_variant:Nn \tl_if_eq:nnT        {V,x}
\cs_generate_variant:Nn \tl_if_eq:nnTF       {o}
\cs_generate_variant:Nn \tl_set:Nn           {cV}
\cs_generate_variant:Nn \tl_replace_all:Nnn  {NV}
\cs_generate_variant:Nn \tl_replace_once:Nnn {Nxx}
\cs_generate_variant:Nn \tl_set_rescan:Nnn   {NnV}
\cs_generate_variant:Nn \prop_item:Nn        {NV}
\cs_generate_variant:Nn \prop_if_in:NnT      {Nx}
\cs_generate_variant:Nn \dim_set:Nn          {NV}
\cs_generate_variant:Nn \skip_set:Nn         {cV}
\cs_generate_variant:Nn \str_if_eq:nnT       {V}

% --------------------------------------------------------------------------
% VARIABLES
% temporary variables:
\tl_new:N  \l__chemformula_tmpa_tl
\tl_new:N  \l__chemformula_tmpb_tl
\tl_new:N  \l__chemformula_tmpc_tl

\dim_new:N \l__chemformula_tmpa_dim
\dim_new:N \l__chemformula_tmpb_dim
\dim_new:N \l__chemformula_tmpc_dim

\int_new:N \l__chemformula_tmpa_int
\int_new:N \l__chemformula_tmpb_int
\int_new:N \l__chemformula_tmpc_int

\box_new:N \l__chemformula_tmpa_box
\box_new:N \l__chemformula_tmpb_box

\seq_new:N \l__chemformula_tmpa_seq

\bool_new:N \l__chemformula_tmpa_bool

% booleans:
% these are official:
\bool_new:N      \l_chemformula_inside_ch_bool
\bool_new:N      \l_chemformula_formal_charges_bool
\bool_new:N      \l_chemformula_formal_chem_bool
\bool_set_true:N \l_chemformula_formal_chem_bool
% those are private:
\bool_new:N      \l__chemformula_options_bool
\bool_new:N      \l__chemformula_is_option_bool
\bool_new:N      \l__chemformula_stoich_bool
\bool_new:N      \l__chemformula_no_stoich_bool
\bool_new:N      \l__chemformula_is_addition_symbol_bool
\bool_new:N      \l__chemformula_is_symbol_bool
\bool_new:N      \l__chemformula_is_arrow_bool
\bool_new:N      \l__chemformula_is_name_bool
\bool_new:N      \l__chemformula_stoich_is_iupac_bool
\bool_new:N      \l__chemformula_stoich_parse_iupac_bool
\bool_new:N      \l__chemformula_first_last_text_bool
\bool_new:N      \l__chemformula_first_last_double_bool
\bool_new:N      \l__chemformula_first_last_single_bool
\bool_new:N      \l__chemformula_first_last_math_bool
\bool_new:N      \l__chemformula_first_last_dollar_bool
\bool_new:N      \l__chemformula_first_last_mathbraces_bool
\bool_new:N      \l__chemformula_number_style_math_bool
\bool_new:N      \l__chemformula_charge_style_math_bool
\bool_new:N      \l__chemformula_is_isotope_bool
\bool_new:N      \l__chemformula_xfrac_bool
\bool_new:N      \l__chemformula_nicefrac_bool
\bool_new:N      \l__chemformula_mathfrac_bool
\bool_set_true:N \l__chemformula_mathfrac_bool
\bool_new:N      \l__chemformula_fss_bool
\bool_new:N      \l__chemformula_fontspec_bool
\bool_new:N      \l__chemformula_fss_family_bool
\bool_new:N      \l__chemformula_fss_series_bool
\bool_new:N      \l__chemformula_fss_shape_bool
\bool_new:N      \l__chemformula_name_width_bool
\bool_new:N      \l__chemformula_charge_full_shift_bool
\bool_new:N      \l__chemformula_tikzexternaldisable_bool
\bool_set_true:N \l__chemformula_tikzexternaldisable_bool
\bool_new:N      \l__chemformula_kroegervink_bool
\bool_new:N      \l__chemformula_inside_subscript_bool
\bool_new:N      \l__chemformula_inside_superscript_bool

% dimensions:
\dim_new:N  \l__chemformula_arrow_length_dim
\dim_new:N  \l__chemformula_arrow_label_height_dim
\dim_new:N  \l__chemformula_arrow_label_offset_dim
\dim_set:Nn \l__chemformula_arrow_label_offset_dim {2pt}
\dim_new:N  \l__chemformula_arrow_minimum_length_dim
\dim_new:N  \l__chemformula_arrow_shortage_dim
\dim_new:N  \l__chemformula_arrow_offset_dim
\dim_new:N  \l__chemformula_arrow_yshift_dim
\dim_set:Nn \l__chemformula_arrow_yshift_dim {0pt}
\dim_new:N  \l__chemformula_radical_radius_dim
\dim_set:Nn \l__chemformula_radical_radius_dim {.2ex}
\dim_new:N  \l__chemformula_radical_hshift_dim
\dim_set:Nn \l__chemformula_radical_hshift_dim {.15em}
\dim_new:N  \l__chemformula_radical_vshift_dim
\dim_set:Nn \l__chemformula_radical_vshift_dim {.5ex}
\dim_new:N  \l__chemformula_radical_space_dim
\dim_set:Nn \l__chemformula_radical_space_dim {.3em}
\dim_new:N  \l__chemformula_arrow_head_dim
\dim_new:N  \l__chemformula_name_dim
\dim_new:N  \l__chemformula_adduct_space_dim
\dim_set:Nn \l__chemformula_adduct_space_dim {.1333em}
\dim_new:N  \l__chemformula_charge_shift_dim
\dim_set:Nn \l__chemformula_charge_shift_dim {.5ex}
\dim_new:N  \l__chemformula_subscript_shift_dim
\dim_new:N  \l__chemformula_superscript_shift_dim
\dim_new:N  \l__chemformula_subscript_dim
\dim_zero:N \l__chemformula_subscript_dim
\dim_new:N  \l__chemformula_superscript_dim
\dim_zero:N \l__chemformula_superscript_dim
\dim_new:N  \l__chemformula_bond_dim
\dim_set:Nn \l__chemformula_bond_dim {.5833em}
\dim_new:N  \l__chemformula_bond_space_dim
\dim_set:Nn \l__chemformula_bond_space_dim {.07em}
\dim_new:N  \l__chemformula_elspec_pair_distance_dim
\dim_set:Nn \l__chemformula_elspec_pair_distance_dim {1ex}
\dim_new:N  \l__chemformula_elspec_pair_line_length_dim
\dim_set:Nn \l__chemformula_elspec_pair_line_length_dim {1.5ex}
\dim_new:N  \l__chemformula_elspec_pair_width_dim
\dim_set:Nn \l__chemformula_elspec_pair_width_dim {1pt}
\dim_new:N  \l__chemformula_kroegervink_positive_radius_dim
\dim_set:Nn \l__chemformula_kroegervink_positive_radius_dim {.3ex}
\dim_new:N  \l__chemformula_kroegervink_positive_hshift_dim
\dim_set:Nn \l__chemformula_kroegervink_positive_hshift_dim {.15em}
\dim_new:N  \l__chemformula_kroegervink_positive_vshift_dim
\dim_set:Nn \l__chemformula_kroegervink_positive_vshift_dim {.5ex}
\dim_new:N  \l__chemformula_kroegervink_positive_space_dim
\dim_set:Nn \l__chemformula_kroegervink_positive_space_dim {.4em}

% skips:
\skip_new:N  \l__chemformula_stoich_space_skip
\skip_set:Nn \l__chemformula_stoich_space_skip { .1667em plus .0333em minus .0117em }
\skip_new:N  \l__chemformula_math_space_skip
\skip_set:Nn \l__chemformula_math_space_skip { .1667em plus .0333em minus .0117em }

% integers:
\int_new:N   \l__chemformula_count_tokens_int
\int_new:N   \g__chemformula_lewis_int

% token lists:
\tl_new:N     \l__chemformula_compound_sep_tl
\tl_set:Nn    \l__chemformula_compound_sep_tl {.5em}
\tl_new:N     \l__chemformula_compound_tl
\tl_new:N     \l__chemformula_arrow_ratio_tl
\tl_set:Nn    \l__chemformula_arrow_ratio_tl {.6}
\tl_new:N     \l__chemformula_radical_style_tl
\tl_new:N     \l__chemformula_arrow_head_tl
\tl_set:Nn    \l__chemformula_arrow_head_tl {cf}
\tl_new:N     \l__chemformula_upper_label_tl
\tl_new:N     \l__chemformula_lower_label_tl
\tl_new:N     \l__chemformula_arrow_style_tl
\tl_new:N     \l__chemformula_arrow_label_style_tl
\tl_set:Nn    \l__chemformula_arrow_label_style_tl { \footnotesize }
\tl_new:N     \l__chemformula_arrow_type_tl
\tl_new:N     \l__chemformula_arrow_penalty_tl
\tl_set:Nn    \l__chemformula_arrow_penalty_tl {0}
\tl_new:N     \l__chemformula_arrow_offset_tl
\tl_set:Nn    \l__chemformula_arrow_offset_tl {.75em}
\tl_new:N     \l__chemformula_bond_penalty_tl
\tl_set:Nn    \l__chemformula_bond_penalty_tl {10000}
\tl_new:N     \l__chemformula_chemformula_tmpa_tl
\tl_new:N     \g__chemformula_options_tl
\tl_new:N     \g__chemformula_output_tl
\tl_new:N     \l__chemformula_input_tl
\tl_new:N     \l__chemformula_stoich_tl
\tl_new:N     \l__chemformula_decimal_output_tl
\tl_set:Nn    \l__chemformula_decimal_output_tl {.}
\tl_new:N     \l__chemformula_font_family_tl
\tl_set_eq:NN \l__chemformula_font_family_tl \f@family
\tl_new:N     \l__chemformula_font_series_tl
\tl_set_eq:NN \l__chemformula_font_series_tl \f@series
\tl_new:N     \l__chemformula_font_shape_tl
\tl_set_eq:NN \l__chemformula_font_shape_tl \f@shape
\tl_new:N     \l__chemformula_format_tl
\tl_new:N     \l__chemformula_atom_format_tl
\tl_new:N     \l__chemformula_fontspec_options_tl
\tl_new:N     \l__chemformula_fontspec_argument_tl
\tl_new:N     \l__chemformula_name_format_tl
\tl_set:Nn    \l__chemformula_name_format_tl { \scriptsize\centering }
\tl_new:N     \l__chemformula_subscript_shift_additional_tl
\tl_set:Nn    \l__chemformula_subscript_shift_additional_tl {0pt}
\tl_new:N     \l__chemformula_superscript_shift_additional_tl
\tl_set:Nn    \l__chemformula_superscript_shift_additional_tl {0pt}
\tl_new:N     \l__chemformula_subscript_tl
\tl_new:N     \l__chemformula_superscript_tl
\tl_new:N     \l__chemformula_elspec_angle_tl
\tl_new:N     \l__chemformula_elspec_radius_tl
\tl_set:Nn    \l__chemformula_elspec_radius_tl {.5ex}
\tl_new:N     \l__chemformula_elspec_default_type_tl
\tl_set:Nn    \l__chemformula_elspec_default_type_tl {pair}
\tl_const:Nx  \c__chemformula_colon_tl { \token_to_str:N : }
\tl_new:N     \l__chemformula_kroegervink_positive_style_tl
\tl_new:N     \l__chemformula_kroegervink_neutral_tl
\tl_set:Nn    \l__chemformula_kroegervink_neutral_tl {$\times$}
\tl_new:N     \l__chemformula_adduct_penalty_tl
\tl_set:Nn    \l__chemformula_adduct_penalty_tl {300}
\tl_new:N     \l__chemformula_catcodes_tl

% property lists:
\prop_new:N \l__chemformula_arrows_code_prop
\prop_new:N \l__chemformula_compound_prop
\prop_new:N \l__chemformula_bonds_prop
\prop_new:N \l__chemformula_stoich_prop
\prop_new:N \l__chemformula_charge_prop
\prop_new:N \l__chemformula_numbers_prop
\prop_new:N \l__chemformula_no_stoich_prop
\prop_new:N \l__chemformula_lewis_elspec_types_prop
\prop_new:N \l__chemformula_kroegervink_prop

% sequences:
\seq_new:N \l__chemformula_arrows_type_seq
\seq_new:N \l__chemformula_input_seq

% boxes:
\box_new:N \l__chemformula_arrow_arg_i_box
\box_new:N \l__chemformula_arrow_arg_ii_box
\box_new:N \l__chemformula_superscript_box
\box_new:N \l__chemformula_subscript_box

% --------------------------------------------------------------------------
% general and helper functions:
\cs_new_protected:Npn \chemformula_tikz:nn #1#2
  {
    \group_begin:
      \chemformula_tikzexternaldisable:
      \tikz [#1] {#2}
    \group_end:
  }
\cs_generate_variant:Nn \chemformula_tikz:nn {xn,nf}

\cs_new_protected:Npn \chemformula_draw:nn #1#2
  { \draw [#1] #2 ;  }
\cs_generate_variant:Nn \chemformula_draw:nn {xn,nx,xx,xf,Vn}

\cs_new_protected:Npn \chemformula_node:nn #1#2
  { \node [#1] {#2}  ;  }
\cs_generate_variant:Nn \chemformula_node:nn {x}

\cs_new_protected:Npn \chemformula_node:nnn #1#2#3
  { \node [#1] (#2) {#3}  ;  }
\cs_generate_variant:Nn \chemformula_node:nnn {nx}

\cs_new_protected:Npn \chemformula_allow_break:
  { \tex_penalty:D \c_zero_int }

\cs_new_protected:Npn \chemformula_leave_vmode:
  { \hbox_unpack:N \c_empty_box }

\cs_new_protected:Npn \chemformula_nobreak:
  { \tex_penalty:D 10000 \scan_stop: }

\cs_new_protected:Npn \chemformula_skip_nobreak:N #1
  {
    \chemformula_nobreak:
    \skip_horizontal:N #1
    \chemformula_nobreak:
  }
\cs_generate_variant:Nn \chemformula_skip_nobreak:N {c}

\cs_new_protected:Npn \chemformula_skip_nobreak:n #1
  {
    \chemformula_nobreak:
    \skip_horizontal:n {#1}
    \chemformula_nobreak:
  }

\cs_new_protected:Npn \chemformula_skip_nobreak_penalty:Nn #1#2
  {
    \chemformula_nobreak:
    \skip_horizontal:N #1
    \tex_penalty:D #2 \scan_stop:
  }
\cs_generate_variant:Nn \chemformula_skip_nobreak_penalty:Nn {c,NV}

\cs_new_protected:Npn \chemformula_dim_to_width:Nn #1#2
  {
    \hbox_set:Nn \l__chemformula_tmpa_box {#2}
    \dim_set:Nn #1 { \box_wd:N \l__chemformula_tmpa_box }
    \box_clear:N \l__chemformula_tmpa_box
  }

% --------------------------------------------------------------------------
% prevent externalisation of tikz parts of the formulae:
\AtBeginDocument { \providecommand \tikzexternaldisable {} }

\keys_define:nn {chemformula}
  {
    tikz-external-disable .bool_set:N  =
      \l__chemformula_tikzexternaldisable_bool
  }

\cs_new_protected:Npn \chemformula_tikzexternaldisable:
  {
    \bool_if:NT \l__chemformula_tikzexternaldisable_bool
      { \tikzexternaldisable }
  }

% --------------------------------------------------------------------------
% circled charge signs:
\cs_new_protected:Npn \chemformula_fplus:
  { \ensuremath { \__chemformula_fplus: } }

\cs_new_protected:Npn \__chemformula_fplus:
  { \mathpalette \__chemformula_fplus_aux: \bigcirc }

\cs_new_protected:Npn \__chemformula_fplus_aux: #1#2
  {
    \ooalign
      {
        \tex_hfil:D
        \c_math_toggle_token
          #1
          \chemformula_plain_plus:
        \c_math_toggle_token
        \tex_hfil:D
        \tex_cr:D
        \tex_hfil:D
        \c_math_toggle_token #1#2 \c_math_toggle_token
        \tex_hfil:D
        \tex_cr:D
      }
  }

\cs_new_protected:Npn \chemformula_fminus:
  { \ensuremath { \__chemformula_fminus: } }

\cs_new_protected:Npn \__chemformula_fminus:
  { \mathpalette \__chemformula_fminus_aux_aux: \bigcirc }

\cs_new_protected:Npn \__chemformula_fminus_aux_aux: #1#2
  {
    \ooalign
      {
        \tex_hfil:D
        \c_math_toggle_token
          #1
          \chemformula_plain_minus:
        \c_math_toggle_token
        \tex_hfil:D
        \tex_cr:D
        \tex_hfil:D
        \c_math_toggle_token #1#2 \c_math_toggle_token
        \tex_hfil:D
        \tex_cr:D
      }
  }

\cs_new_protected:Npn \chemformula_plain_plus:
  {
    \bool_if:NTF \l__chemformula_charge_style_math_bool
      { \ensuremath {+} }
      { \mode_if_math:TF { \text {+} } {+} }
  }

\cs_new_protected:Npn \chemformula_formal_plus:
  {
    \bool_if:NTF \l_chemformula_formal_chem_bool
      { \chemformula_fplus: }
      {
        \mode_if_math:TF
          { \oplus }
          { \c_math_toggle_token \oplus \c_math_toggle_token }
      }
  }

\cs_new_protected:Npn \chemformula_plus:
  {
    \bool_if:NTF \l_chemformula_formal_charges_bool
      { \chemformula_formal_plus: }
      { \chemformula_plain_plus: }
  }

\cs_new_protected:Npn \chemformula_plain_minus:
  {
    \bool_if:NTF \l__chemformula_charge_style_math_bool
      { \ensuremath {-} }
      { \mode_if_math:TF { \text { \textendash } } { \textendash } }
  }

\cs_new_protected:Npn \chemformula_formal_minus:
  {
    \bool_if:NTF \l_chemformula_formal_chem_bool
      { \chemformula_fminus: }
      {
        \mode_if_math:TF
          { \ominus }
          { \c_math_toggle_token \ominus \c_math_toggle_token }
      }
  }
  
\cs_new_protected:Npn \chemformula_minus:
  {
     \bool_if:NTF \l_chemformula_formal_charges_bool
      { \chemformula_formal_minus: }
      { \chemformula_plain_minus: }
  }

% --------------------------------------------------------------------------
% Some basic internal commands:

\cs_new_protected:Npn \chemformula_radical_or_decimal:
  {
    \peek_meaning:NTF \c_group_begin_token
      { \chemformula_decimal: }
      { \chemformula_radical: }
  }

\cs_new_protected:Npn \chemformula_radical:
  {
    \hbox_overlap_right:n
      {
        \chemformula_tikz:xn
          {
            \l__chemformula_radical_style_tl ,
            baseline ,
            minimum~ height = 0pt ,
            minimum~ width = 0pt ,
            inner~ sep = 0pt ,
            outer~ sep = 0pt ,
            overlay
          }
          {
            \fill (0,0)
              ++(
                \l__chemformula_radical_hshift_dim ,
                \l__chemformula_radical_vshift_dim
              )
              circle (\l__chemformula_radical_radius_dim) ;
          }
      }
    \chemformula_skip_nobreak:N \l__chemformula_radical_space_dim
  }

\keys_define:nn {chemformula}
  {
    % TikZ style:
    radical-style  .tl_set:N  = \l__chemformula_radical_style_tl ,
    radical-radius .dim_set:N = \l__chemformula_radical_radius_dim ,
    % shift before:
    radical-hshift .dim_set:N = \l__chemformula_radical_hshift_dim ,
    radical-vshift .dim_set:N = \l__chemformula_radical_vshift_dim ,
    % space after:
    radical-space  .dim_set:N = \l__chemformula_radical_space_dim
  }

\cs_new_protected:Npn \chemformula_adduct:
  {
    \chemformula_skip_nobreak:N \l__chemformula_adduct_space_dim
    \textperiodcentered
    \chemformula_skip_nobreak_penalty:NV
      \l__chemformula_adduct_space_dim
      \l__chemformula_adduct_penalty_tl
  }
\cs_new_eq:NN \chemformula_star_adduct: \chemformula_adduct:

% --------------------------------------------------------------------------
% Kroeger-Vink notation
\cs_new_protected:Npn \chemformula_kroegervink_negative:
  { \ensuremath { \prime } }

\cs_new_protected:Npn \chemformula_kroegervink_neutral:
  { \ensuremath { \text {\l__chemformula_kroegervink_neutral_tl } } }

\cs_new_protected:Npn \chemformula_kroegervink_positive:
  {
    \hbox_overlap_right:n
      {
        \chemformula_tikz:xn
          {
            \l__chemformula_kroegervink_positive_style_tl ,
            baseline ,
            minimum~ height = 0pt ,
            minimum~ width = 0pt ,
            inner~ sep = 0pt ,
            outer~ sep = 0pt ,
            overlay
          }
          {
            \fill (0,0)
              ++(
                \l__chemformula_kroegervink_positive_hshift_dim ,
                \l__chemformula_kroegervink_positive_vshift_dim
              )
              circle (\l__chemformula_kroegervink_positive_radius_dim) ;
          }
      }
    \chemformula_skip_nobreak:N \l__chemformula_kroegervink_positive_space_dim
  }

\cs_new_protected:Npn \chemformula_prime:
  {
    \bool_if:NTF \l__chemformula_kroegervink_bool
      { \chemformula_kroegervink_negative: }
      {'}
  }

\keys_define:nn {chemformula}
  {
    kroeger-vink       .bool_set:N =
      \l__chemformula_kroegervink_bool ,
    % TikZ style:
    kv-positive-style  .tl_set:N   =
      \l__chemformula_kroegervink_positive_style_tl ,
    kv-positive-radius .dim_set:N  =
      \l__chemformula_kroegervink_positive_radius_dim ,
    % spacing:
    kv-positive-hshift .dim_set:N  =
      \l__chemformula_kroegervink_positive_hshift_dim ,
    kv-positive-vshift .dim_set:N  =
      \l__chemformula_kroegervink_positive_vshift_dim ,
    kv-positive-offset .dim_set:N  =
      \l__chemformula_kroegervink_positive_space_dim ,
    % symbol:
    kv-neutral-symbol  .tl_set:N   =
      \l__chemformula_kroegervink_neutral_tl
  }

% --------------------------------------------------------------------------
% ARROW DEFINITION
% arrow heads:

% full tip for pairs
\pgfarrowsdeclare { cf~internal } { cf~internal }
  {
    \dim_set:Nn \l__chemformula_arrow_head_dim
      { 2.5pt + 2.5\pgflinewidth }
    \pgfarrowsleftextend { -\l__chemformula_arrow_head_dim }
    \pgfarrowsrightextend { .5\pgflinewidth }
  }
  {
    \dim_set:Nn \l__chemformula_arrow_head_dim
      { 2.5pt + 2.5\pgflinewidth }
    \pgfsetdash {} { 0pt }
    \pgfsetroundjoin
    \pgfsetroundcap
    \pgfpathmoveto { \pgfpoint { 0pt } { 0pt } }
    \pgfpathlineto
      {
        \pgfpoint
          { -\l__chemformula_arrow_head_dim }
          { .3\l__chemformula_arrow_head_dim }
      }
    \pgfpathlineto
      { \pgfpoint { -.5\l__chemformula_arrow_head_dim } { 0pt } }
    \pgfpathlineto
      {
        \pgfpoint
          { -\l__chemformula_arrow_head_dim }
          { -.3\l__chemformula_arrow_head_dim }
      }
    \pgfpathlineto { \pgfpoint { 0pt } { 0pt } }
    \pgfusepathqfillstroke
  }

% half tip on the left
\pgfarrowsdeclare { left~cf~internal } { left~cf~internal }
  {
    \dim_set:Nn \l__chemformula_arrow_head_dim
      { 2.5pt + 2.5\pgflinewidth }
    \pgfarrowsleftextend { -\l__chemformula_arrow_head_dim }
    \pgfarrowsrightextend { .5\pgflinewidth }
  }
  {
    \dim_set:Nn \l__chemformula_arrow_head_dim
      { 2.5pt + 2.5\pgflinewidth }
    \pgfsetdash {} { 0pt }
    \pgfsetroundjoin
    \pgfsetroundcap
    \pgfpathmoveto { \pgfpoint { 0pt } { 0pt } }
    \pgfpathlineto
      {
        \pgfpoint
          { -\l__chemformula_arrow_head_dim }
          { .3\l__chemformula_arrow_head_dim }
      }
    \pgfpathlineto { \pgfpoint { -.5\l__chemformula_arrow_head_dim } { 0pt } }
    \pgfpathlineto { \pgfpoint { 0pt } { 0pt } }
    \pgfusepathqfillstroke
  }

% half tip in the right
\pgfarrowsdeclare { right~cf~internal } { right~cf~internal }
  {
    \dim_set:Nn \l__chemformula_arrow_head_dim
      { 2.5pt + 2.5\pgflinewidth }
    \pgfarrowsleftextend { -\l__chemformula_arrow_head_dim }
    \pgfarrowsrightextend { .5\pgflinewidth }
  }
  {
    \dim_set:Nn \l__chemformula_arrow_head_dim
      { 2.5pt + 2.5\pgflinewidth }
    \pgfsetdash {} { 0pt }
    \pgfsetroundjoin
    \pgfsetroundcap
    \pgfpathmoveto { \pgfpoint { 0pt } { 0pt } }
    \pgfpathlineto
      {
        \pgfpoint
          { -\l__chemformula_arrow_head_dim }
          { -.3\l__chemformula_arrow_head_dim }
      }
    \pgfpathlineto { \pgfpoint { -.5\l__chemformula_arrow_head_dim } { 0pt } }
    \pgfpathlineto { \pgfpoint { 0pt } { 0pt } }
    \pgfusepathqfillstroke
  }

\pgfkeys{
  cf~       /.tip = {cf~internal} ,
  left~cf~  /.tip = {left~cf~internal} ,
  right~cf~ /.tip = {right~cf~internal}
}

% --------------------------------------------------------------------------
% determine length in dependency of labels
\cs_new_protected:Npn \__chemformula_determine_arrow_length:NN #1#2
  {
    \dim_set:NV \l__chemformula_arrow_offset_dim \l__chemformula_arrow_offset_tl
    \box_set_eq:NN \l__chemformula_tmpa_box #1
    \dim_set:Nn \l__chemformula_tmpa_dim { \box_wd:N \l__chemformula_tmpa_box }
    \box_set_eq:NN \l__chemformula_tmpa_box #2
    \dim_set:Nn \l__chemformula_tmpb_dim { \box_wd:N \l__chemformula_tmpa_box }
    \box_clear:N \l__chemformula_tmpa_box
    \dim_compare:nTF { \l__chemformula_tmpa_dim >= \l__chemformula_tmpb_dim }
      { \dim_set_eq:NN \l__chemformula_arrow_length_dim \l__chemformula_tmpa_dim }
      { \dim_set_eq:NN \l__chemformula_arrow_length_dim \l__chemformula_tmpb_dim }
    \dim_add:Nn \l__chemformula_arrow_length_dim
      { 2\l__chemformula_arrow_offset_dim }
    \dim_compare:nF
      { \l__chemformula_arrow_length_dim > \l__chemformula_arrow_minimum_length_dim }
      {
        \dim_set_eq:NN
          \l__chemformula_arrow_length_dim
          \l__chemformula_arrow_minimum_length_dim
      }
    \dim_set:Nn \l__chemformula_arrow_shortage_dim
      {
        (
          \l__chemformula_arrow_length_dim
          -
          \l__chemformula_arrow_length_dim *
          \dim_ratio:nn { \l__chemformula_arrow_ratio_tl pt } { 1pt }
        ) * 1/2
      }
    \box_set_eq:NN \l__chemformula_tmpa_box #1
    \dim_set:Nn \l__chemformula_tmpa_dim
      { \box_ht:N \l__chemformula_tmpa_box + \box_dp:N \l__chemformula_tmpa_box }
    \box_set_eq:NN \l__chemformula_tmpa_box #2
    \dim_set:Nn \l__chemformula_tmpb_dim
      { \box_ht:N \l__chemformula_tmpa_box + \box_dp:N \l__chemformula_tmpa_box }
    \box_clear:N \l__chemformula_tmpa_box
    \dim_compare:nTF { \l__chemformula_tmpa_dim >= \l__chemformula_tmpb_dim }
      { \dim_set_eq:NN \l__chemformula_arrow_label_height_dim \l__chemformula_tmpa_dim }
      { \dim_set_eq:NN \l__chemformula_arrow_label_height_dim \l__chemformula_tmpb_dim }
  }

% --------------------------------------------------------------------------
% define arrow types

% draw arrows:
\cs_new_protected:Npn \__chemformula_draw_arrow:nnn #1#2#3
  {
    \chemformula_if_is_arrow:nTF {#1}
      {
        \prop_get:NnN \l__chemformula_arrows_code_prop
          {#1}
          \l__chemformula_tmpa_tl
        \__chemformula_arrow_draw:Vnn \l__chemformula_tmpa_tl {#2} {#3}
      }
      { \msg_error:nnn {chemformula} {unknown-arrow} {#1} }
  }

% this will be \arrow inside \ch
\DeclareDocumentCommand \charrow { mO{}O{} }
  {
    \group_begin:
      \__chemformula_draw_arrow:nnn {#1} {#2} {#3}
    \group_end:
  }

% draw the arrows
\cs_new_protected:Npn \__chemformula_arrow_draw:nnn #1#2#3
  {
    \chemformula_nobreak:
    % prepare arrow code for drawing:
    \tl_set_rescan:Nnn \l__chemformula_tmpc_tl
      { \char_set_catcode_letter:N \_ } {#1}
    % determine length of the arrow
    \hbox_set:Nn \l__chemformula_arrow_arg_i_box
      { \tl_use:N \l__chemformula_arrow_label_style_tl #2 }
    \hbox_set:Nn \l__chemformula_arrow_arg_ii_box
      { \tl_use:N \l__chemformula_arrow_label_style_tl #3 }
    \__chemformula_determine_arrow_length:NN
      \l__chemformula_arrow_arg_i_box
      \l__chemformula_arrow_arg_ii_box
    \chemformula_tikz:nf
      { inner~sep=0, baseline=(chemformula_arrow_start.base) }
      {
        % the coordinates
        \chemformula_draw:xn
          { \l__chemformula_arrow_style_tl }
          {
            coordinate (chemformula_arrow_start) (0pt,0pt)
            ++ ( \l__chemformula_compound_sep_tl , .3432em)
            ++ (0pt,\l__chemformula_arrow_yshift_dim)
            coordinate (cf_arrow_start)
            ++ ( .5 * \dim_use:N \l__chemformula_arrow_length_dim , 0pt)
            coordinate (cf_arrow_mid)
            ++ ( .5 * \dim_use:N \l__chemformula_arrow_length_dim , 0pt)
            coordinate (cf_arrow_end)
            ++ ( \l__chemformula_compound_sep_tl , 0pt )
            (cf_arrow_start) ++ (\l__chemformula_arrow_shortage_dim , 0pt)
            coordinate (cf_arrow_mid_start)
            (cf_arrow_end) ++ (-\l__chemformula_arrow_shortage_dim , 0pt)
            coordinate (cf_arrow_mid_end)
          }
        % the arrow labels:
        \draw
            node
              [ above=\l__chemformula_arrow_label_offset_dim ] at (cf_arrow_mid)
              { \box_use:N \l__chemformula_arrow_arg_i_box }
            node
              [ below=\l__chemformula_arrow_label_offset_dim ] at (cf_arrow_mid)
              { \box_use:N \l__chemformula_arrow_arg_ii_box }
          ;
        \tl_use:N \l__chemformula_tmpc_tl
      }
    \chemformula_skip_nobreak_penalty:NV
      \c_zero_skip
      \l__chemformula_arrow_penalty_tl
  }
\cs_generate_variant:Nn \__chemformula_arrow_draw:nnn {V}

\prg_new_conditional:Npnn \chemformula_if_is_arrow:n #1 {p,T,F,TF}
  {
    \seq_if_in:NnTF \l__chemformula_arrows_type_seq {#1}
      { \prg_return_true: }
      { \prg_return_false: }
  }

% commands to declare arrows (can also be used by users):
%  #1: symbol, #2: code
\NewDocumentCommand \NewChemArrow {mm}
  {
    \chemformula_if_is_arrow:nTF {#1}
      { \msg_error:nnn {chemformula} {new-arrow} {#1} }
      {
        % store type in the sequence and code in the property list
        \seq_put_right:Nn \l__chemformula_arrows_type_seq {#1}
        \prop_put:Nnn \l__chemformula_arrows_code_prop {#1} {#2}
      }
  }

\NewDocumentCommand \DeclareChemArrow {mm}
  {
    \chemformula_if_is_arrow:nF {#1}
      { \seq_put_right:Nn \l__chemformula_arrows_type_seq {#1} }
    % store type in the sequence and code in the property list
    \prop_put:Nnn \l__chemformula_arrows_code_prop {#1} {#2}
  }

\NewDocumentCommand \RenewChemArrow {mm}
  {
    \chemformula_if_is_arrow:nTF {#1}
      { \prop_put:Nnn \l__chemformula_arrows_code_prop {#1} {#2} }
      { \msg_error:nnn {chemformula} {renew-arrow} {#1} }
  }

\NewDocumentCommand \ProvideChemArrow {mm}
  {
    \chemformula_if_is_arrow:nF {#1}
      {
        % store type in the sequence and code in the property list
        \seq_put_right:Nn \l__chemformula_arrows_type_seq {#1}
        \prop_put:Nnn \l__chemformula_arrows_code_prop {#1} {#2}
      }
  }
  
\NewDocumentCommand \ShowChemArrow {m}
  {
    \chemformula_if_is_arrow:nT {#1}
      {
        \prop_get:NnN \l__chemformula_arrows_code_prop {#1}
          \l__chemformula_tmpa_tl
        \tl_to_str:N \l__chemformula_tmpa_tl
      }
  }

% declare arrows:
% standard arrows:
\NewChemArrow {->}
  { \draw[chemarrow,-cf] (cf_arrow_start) -- (cf_arrow_end) ; }
\NewChemArrow {<-}
  { \draw[chemarrow,cf-] (cf_arrow_start) -- (cf_arrow_end) ; }
\NewChemArrow {<->}
  { \draw[chemarrow,cf-cf] (cf_arrow_start) -- (cf_arrow_end) ; }
% equilibria:
\NewChemArrow {<=>}
  {
    \draw[chemarrow,-left~cf]
      ([yshift=.15ex]cf_arrow_start) -- ([yshift=.15ex]cf_arrow_end) ;
    \draw[chemarrow,left~cf-]
      ([yshift=-.15ex]cf_arrow_start) -- ([yshift=-.15ex]cf_arrow_end) ;
  }
\NewChemArrow {<=>>}
  {
    \draw[chemarrow,-left~cf]
      ([yshift=.15ex]cf_arrow_start) -- ([yshift=.15ex]cf_arrow_end) ;
    \draw[chemarrow,left~cf-]
      ([yshift=-.15ex]cf_arrow_mid_start) -- ([yshift=-.15ex]cf_arrow_mid_end) ;
  }
\NewChemArrow {<<=>}
  {
    \draw[chemarrow,-left~cf]
      ([yshift=.15ex]cf_arrow_mid_start) -- ([yshift=.15ex]cf_arrow_mid_end) ;
    \draw[chemarrow,left~cf-]
      ([yshift=-.15ex]cf_arrow_start) -- ([yshift=-.15ex]cf_arrow_end) ;
  }
% two arrows:
\NewChemArrow {<>}
  {
    \draw[chemarrow,-cf]
      ([yshift=.2ex]cf_arrow_start) -- ([yshift=.2ex]cf_arrow_end) ;
    \draw[chemarrow,cf-,]
      ([yshift=-.2ex]cf_arrow_start) -- ([yshift=-.2ex]cf_arrow_end) ;
  }
% isolobal arrow:
\NewChemArrow {<o>}
  {
    \draw[chemarrow,cf-cf] (cf_arrow_start) -- (cf_arrow_end) ;
    \draw[chemarrow]
      (cf_arrow_mid) .. controls ++(-.9ex,-1.5ex) and
      ++(.9ex,-1.5ex) .. (cf_arrow_mid) ;
  }
% equilibria upside down:
\NewChemArrow {>>=<}
  {
    \draw[chemarrow,right~cf-]
      ([yshift=.15ex]cf_arrow_start) -- ([yshift=.15ex]cf_arrow_end) ;
    \draw[chemarrow,-right~cf]
      ([yshift=-.15ex]cf_arrow_mid_start) -- ([yshift=-.15ex]cf_arrow_mid_end) ;
  }
\NewChemArrow {>=<<}
  {
    \draw[chemarrow,right~cf-]
      ([yshift=.15ex]cf_arrow_mid_start) -- ([yshift=.15ex]cf_arrow_mid_end) ;
    \draw[chemarrow,-right~cf]
      ([yshift=-.15ex]cf_arrow_start) -- ([yshift=-.15ex]cf_arrow_end) ;
  }
\NewChemArrow {>=<}
  {
    \draw[chemarrow,right~cf-]
      ([yshift=.15ex]cf_arrow_start) -- ([yshift=.15ex]cf_arrow_end) ;
    \draw[chemarrow,-right~cf]
      ([yshift=-.15ex]cf_arrow_start) -- ([yshift=-.15ex]cf_arrow_end) ;
  }
%
\NewChemArrow {<==>}
  {
    \draw[chemarrow,double~ equal~ sign~ distance,double,Implies-Implies]
      (cf_arrow_start) -- (cf_arrow_end) ;
  }
% no reaction:
\NewChemArrow {-/>}
  {
    \draw[chemarrow,-cf] (cf_arrow_start) -- (cf_arrow_end) ;
    \draw[chemarrow] (cf_arrow_mid) ++ (.2ex,.4ex) -- ++(-.4ex,-.8ex) ;
    \draw[chemarrow] (cf_arrow_mid) ++ (-.2ex,.4ex) -- ++(-.4ex,-.8ex) ;
  }
\NewChemArrow {</-}
  {
    \draw[chemarrow,cf-] (cf_arrow_start) -- (cf_arrow_end) ;
    \draw[chemarrow] (cf_arrow_mid) ++ (.2ex,.4ex) -- ++(-.4ex,-.8ex) ;
    \draw[chemarrow] (cf_arrow_mid) ++ (.6ex,.4ex) -- ++(-.4ex,-.8ex) ;
  }
% net reaction:
\NewChemArrow {==} { \node at ([yshift=-.2ex]cf_arrow_mid) { = } ; }
% quasi equilibria:
\NewChemArrow{<=o>}{
  \draw[chemarrow,-left~cf]
    ([yshift=.15ex]cf_arrow_start) -- ([yshift=.15ex]cf_arrow_end) ;
  \draw[chemarrow,left~cf-]
    ([yshift=-.15ex]cf_arrow_start) -- ([yshift=-.15ex]cf_arrow_end) ;
  \draw[chemarrow] (cf_arrow_mid) circle (.4ex) ;
}
\NewChemArrow{<=o>>}{
  \draw[chemarrow,-left~cf]
    ([yshift=.15ex]cf_arrow_start) -- ([yshift=.15ex]cf_arrow_end) ;
  \draw[chemarrow,left~cf-]
    ([yshift=-.15ex]cf_arrow_mid_start) -- ([yshift=-.15ex]cf_arrow_mid_end) ;
  \draw[chemarrow] (cf_arrow_mid) circle (.4ex) ;
}
\NewChemArrow{<<=o>}{
  \draw[chemarrow,-left~cf]
    ([yshift=.15ex]cf_arrow_mid_start) -- ([yshift=.15ex]cf_arrow_mid_end) ;
  \draw[chemarrow,left~cf-]
    ([yshift=-.15ex]cf_arrow_start) -- ([yshift=-.15ex]cf_arrow_end) ;
  \draw[chemarrow] (cf_arrow_mid) circle (.4ex) ;
}

% --------------------------------------------------------------------------
% arrow customization
\keys_define:nn {chemformula}
  {
    arrow-offset     .tl_set:N  = \l__chemformula_arrow_offset_tl ,
    arrow-min-length .dim_set:N = \l__chemformula_arrow_minimum_length_dim ,
    arrow-yshift     .dim_set:N = \l__chemformula_arrow_yshift_dim ,
    arrow-ratio      .tl_set:N  = \l__chemformula_arrow_ratio_tl ,
    label-offset     .dim_set:N = \l__chemformula_arrow_label_offset_dim ,
    compound-sep     .tl_set:N  = \l__chemformula_compound_sep_tl ,
    arrow-style      .tl_set:N  = \l__chemformula_arrow_style_tl ,
    label-style      .tl_set:N  = \l__chemformula_arrow_label_style_tl ,
    arrow-penalty    .tl_set:N  = \l__chemformula_arrow_penalty_tl ,
    arrow-style      .code:n    =  \tikzset { chemarrow/.style={#1} } ,
  }

% --------------------------------------------------------------------------
% MAIN COMMAND
\cs_new_protected:Npn \__chemformula_ch_main:n #1
  {
    \group_begin:
      % abort if `#1' is blank:
      \tl_if_blank:nF {#1}
        {
          \tl_set:Nn \l__chemmacros_tmpa_tl {#1}
          \tl_trim_spaces:N \l__chemmacros_tmpa_tl
          \seq_set_split:NnV \l__chemformula_input_seq {~} \l__chemmacros_tmpa_tl
          % \seq_show:N \l__chemformula_input_seq
          % remove empty items in order to avoid unnecessary processing:
          \seq_remove_all:Nn \l__chemformula_input_seq {}
          % disable \xspace:
          \cs_set_eq:NN \xspace \scan_stop:
          \bool_set_true:N \l_chemformula_inside_ch_bool
          \tl_if_blank:VF \g__chemformula_options_tl
            {
              \keys_set:nV
                {chemformula}
                \g__chemformula_options_tl
            }
          \seq_map_function:NN
            \l__chemformula_input_seq
            \__chemformula_input:n
          % \tl_analysis_show:N \l__chemformula_input_tl
          \__chemformula_prepare_output:NV
            \l__chemformula_input_tl
            \l__chemformula_catcodes_tl
          % \tl_analysis_show:N \l__chemformula_input_tl
          \chemformula_write:V \l__chemformula_input_tl
        }
    \group_end:
  }

\cs_new_protected:Npn \chemformula_write:n #1
  {
    \mode_if_math:TF
      { \text { \chemformula_font_inner: #1} }
      { { \chemformula_font_inner: #1 } }
  }
\cs_generate_variant:Nn \chemformula_write:n {V}

% internal version of \ch command:
\cs_new_protected:Npn \chemformula_ch:nn #1#2
  {
    \group_align_safe_begin:
      \chemformula_leave_vmode:
      \tl_if_blank:nTF {#1}
        { \tl_gclear:N \g__chemformula_options_tl }
        { \tl_gset:Nn  \g__chemformula_options_tl {#1} }
      \__chemformula_ch_newline:w #2 \\ \q_no_value \q_stop
    \group_align_safe_end:
    \tl_use:N    \g__chemformula_output_tl
%    \tl_show:N   \g__chemformula_output_tl
    \tl_gclear:N \g__chemformula_output_tl
  }
\cs_generate_variant:Nn \chemformula_ch:nn {V,nV,VV}

% Trickery to use \ch inside align environment:
% bypass \\:
\cs_new_protected:Npn \__chemformula_ch_newline:w #1 \\ #2 \q_stop
  {
    \__chemformula_ch_align:w #1 & \q_no_value \q_stop
    \quark_if_no_value:nF {#2}
      {
        \tl_gput_right:Nn \g__chemformula_output_tl { \\ }
        % look for optional argument:
        \peek_meaning:NTF [
          { \__chemformula_ch_newline_skip:w }
          { \__chemformula_ch_newline:w }
        #2 \q_stop
      }
  }

\cs_new_protected:Npn \__chemformula_ch_newline_skip:w [#1]
  {
    \tl_gput_right:Nn \g__chemformula_output_tl { [#1] }
    \__chemformula_ch_newline:w
  }

% bypass &:
\cs_new_protected:Npn \__chemformula_ch_align:w #1 & #2 \q_stop
  {
    \__chemformula_ch_intertext:w #1 \intertext{} \q_no_value \q_stop
    \quark_if_no_value:nF {#2}
      {
        \tl_gput_right:Nn \g__chemformula_output_tl {&}
        \__chemformula_ch_align:w #2 \q_stop
      } 
  }

% bypass \intertext:
\cs_new_protected:Npn \__chemformula_ch_intertext:w #1 \intertext#2 #3 \q_stop
  {
    \__chemformula_ch_shortintertext:w #1 \shortintertext{} \q_no_value \q_stop
    \quark_if_no_value:nF {#3}
      {
        \tl_gput_right:Nn \g__chemformula_output_tl { \intertext {#2} }
        \__chemformula_ch_intertext:w #3 \q_stop
      }
  }

% bypass \shortintertext:
\cs_new_protected:Npn \__chemformula_ch_shortintertext:w #1 \shortintertext#2 #3 \q_stop
  {
    \tl_gput_right:Nn \g__chemformula_output_tl
      { \__chemformula_ch_main:n {#1} }
    \quark_if_no_value:nF {#3}
      {
        \tl_gput_right:Nn \g__chemformula_output_tl { \shortintertext {#2} }
        \__chemformula_ch_shortintertext:w #3 \q_stop
      }
  }

% --------------------------------------------------------------------------
% user commands:
\NewDocumentCommand \ch { O{}m }
  { \chemformula_ch:nn {#1} {#2} }

\cs_new_protected:Npn \chemformula_chcpd:nn #1#2
  {
    \group_begin:
      \tl_if_blank:nF {#2}
        {
          \keys_set:nn {chemformula} {#1}
          \__chemformula_save_catcodes:
          \__chemformula_sanitize:Nn
            \l__chemformula_chemformula_tmpa_tl
            {#2}
          \__chemformula_input_compound_no_check:NV
            \l__chemformula_compound_tl
            \l__chemformula_chemformula_tmpa_tl
          \__chemformula_prepare_output:NV
            \l__chemformula_compound_tl
            \l__chemformula_catcodes_tl
          \chemformula_write:V \l__chemformula_compound_tl
        }
    \group_end:
  }

\NewDocumentCommand \chcpd { O{}m }
  { \chemformula_chcpd:nn {#1} {#2} }

% --------------------------------------------------------------------------
% arrow generation & detection
\cs_new_protected:Npn \__chemformula_generate_arrows:Nn #1#2
  {
    % TODO: is the rescan necessary? Can't we use strings?
    \tl_set_rescan:Nnn \l__chemformula_tmpa_tl
      {
        \char_set_catcode_other:N \>
        \char_set_catcode_other:N \<
        \char_set_catcode_other:N \-
        \char_set_catcode_other:N \=
      }
      {#2}
    \int_zero:N \l__chemformula_tmpa_int
    \seq_clear:N \l__chemformula_tmpa_seq
    \seq_map_inline:Nn \l__chemformula_arrows_type_seq
      {
        \tl_if_in:VnT \l__chemformula_tmpa_tl {##1}
          {
            \int_incr:N \l__chemformula_tmpa_int
            \seq_put_right:Nn \l__chemformula_tmpa_seq {##1}
          }
      }
    \int_compare:nTF { \l__chemformula_tmpa_int = 1 }
      {
        \tl_replace_once:Nxx \l__chemformula_tmpa_tl
          { \seq_item:Nn \l__chemformula_tmpa_seq {1} }
          { \exp_not:N \charrow { \seq_item:Nn \l__chemformula_tmpa_seq {1} } }
      }
      {
        \int_zero:N \l__chemformula_tmpa_int
        \int_zero:N \l__chemformula_tmpb_int
        \int_zero:N \l__chemformula_tmpc_int
        \seq_map_inline:Nn \l__chemformula_tmpa_seq
          {
            \int_incr:N \l__chemformula_tmpa_int
            \int_compare:nT { \l__chemformula_tmpb_int < \tl_count:n {##1} }
              {
                \int_set:Nn \l__chemformula_tmpb_int { \tl_count:n {##1} }
                \int_set_eq:NN \l__chemformula_tmpc_int \l__chemformula_tmpa_int
              }
          }
        \tl_replace_once:Nxx \l__chemformula_tmpa_tl
          { \seq_item:Nn \l__chemformula_tmpa_seq {\l__chemformula_tmpc_int} }
          {
            \exp_not:N \charrow
              {
                \seq_item:Nn \l__chemformula_tmpa_seq
                  { \l__chemformula_tmpc_int }
              }
          }
      }
    \tl_set:NV #1 \l__chemformula_tmpa_tl
  }
\cs_generate_variant:Nn \__chemformula_generate_arrows:Nn {NV}

\cs_new_protected:Npn \__chemformula_detect_arrows:n #1
  {
    \bool_set_false:N \l__chemformula_is_arrow_bool
    \prop_map_inline:Nn \l__chemformula_arrows_code_prop
      {
        \__chemformula_sanitize:Nn \l__chemformula_tmpa_tl {##1}
        \tl_if_in:nVT {#1} \l__chemformula_tmpa_tl
          {
            \bool_set_true:N \l__chemformula_is_arrow_bool
            \prop_map_break:
          }
      }
  }

% --------------------------------------------------------------------------
% compound names
\cs_new_protected:Npn \__chemformula_name_compound:w
  { \peek_meaning:NTF ( { \chname } { ! } }

\NewDocumentCommand \chname { R(){}R(){} }
  { \chemformula_chwritebelow:nn {#1} {#2} }

\cs_new_protected:Npn \chemformula_chwritebelow:nn #1#2
  {
    % \tl_set:Nn \l_tmpa_tl {#1} \show \l_tmpa_tl
    % \tl_set:Nn \l_tmpa_tl {#2} \show \l_tmpa_tl
    \c_math_toggle_token
      \underset
        {
          \bool_if:NF \l__chemformula_name_width_bool
            {
              \chemformula_dim_to_width:Nn \l__chemformula_name_dim
                {
                  \chemformula_group_begin:
                  \l__chemformula_name_format_tl {#1}
                  \chemformula_group_end:
                }
            }
          \parbox
            { \dim_use:N \l__chemformula_name_dim }
            {
              \chemformula_group_begin:
              \l__chemformula_name_format_tl {#1}
              \chemformula_group_end:
            }
        }
        {
          \text
            {
              \chemformula_group_begin:
              \strut #2
              \chemformula_group_end:
            }
        }
    \c_math_toggle_token
  }

\cs_new_protected:Npn \__chemformula_generate_name:Nn #1#2
  {
    \__chemformula_sanitize:Nn \l__chemformula_tmpa_tl {#2}
    \tl_if_in:VnT \l__chemformula_tmpa_tl {!}
      {
        \tl_replace_all:Nnn \l__chemformula_tmpa_tl
          {!}
          { \__chemformula_name_compound:w }
      }
    \tl_set:NV #1 \l__chemformula_tmpa_tl
  }
\cs_generate_variant:Nn \__chemformula_generate_name:Nn {NV}

\cs_new_protected:Npn \__chemformula_detect_name:n #1
  {
     \bool_set_false:N \l__chemformula_is_name_bool
     \tl_set:Nn \l__chemformula_tmpa_tl {#1}
     \tl_if_in:VnT \l__chemformula_tmpa_tl { ! }
       { \bool_set_true:N \l__chemformula_is_name_bool }
  }

% --------------------------------------------------------------------------
% parsing input
% compounds:
\cs_new_protected:Npn \chemformula_new_compound_property:nn #1#2
  {
    \prop_if_in:NnTF \l__chemformula_compound_prop {#1}
      { \msg_error:nnn {chemformula} {new-compound-property} {#1} }
      { \prop_put:Nnn \l__chemformula_compound_prop {#1} {#2} }
  }
  
\cs_new_protected:Npn \chemformula_renew_compound_property:nn #1#2
  {
    \prop_if_in:NnTF \l__chemformula_compound_prop {#1}
      { \prop_put:Nnn \l__chemformula_compound_prop {#1} {#2} }
      { \msg_error:nnn {chemformula} {renew-compound-property} {#1} }
  }

\cs_new_protected:Npn \chemformula_provide_compound_property:nn #1#2
  { \prop_put_if_new:Nnn \l__chemformula_compound_prop {#1} {#2} }
  
\cs_new_protected:Npn \chemformula_declare_compound_property:nn #1#2
  { \prop_put:Nnn \l__chemformula_compound_prop {#1} {#2} }

\cs_new_protected:Npn \chemformula_remove_compound_property:n #1
  { \prop_remove:Nn \l__chemformula_compound_prop {#1} }

\NewDocumentCommand \NewChemCompoundProperty {mm}
  { \chemformula_new_compound_property:nn {#1} {#2} }

\NewDocumentCommand \RenewChemCompoundProperty {mm}
  { \chemformula_renew_compound_property:nn {#1} {#2} }

\NewDocumentCommand \ProvideChemCompoundProperty {mm}
  { \chemformula_provide_compound_property:nn {#1} {#2} }
  
\NewDocumentCommand \DeclareChemCompoundProperty {mm}
  { \chemformula_declare_compound_property:nn {#1} {#2} }

\NewDocumentCommand \RemoveChemCompoundProperty {m}
  { \chemformula_remove_compound_property:n {#1} }
  
\chemformula_new_compound_property:nn { 0 } { \chemformula_subscript:n { 0 } }
\chemformula_new_compound_property:nn { 1 } { \chemformula_subscript:n { 1 } }
\chemformula_new_compound_property:nn { 2 } { \chemformula_subscript:n { 2 } }
\chemformula_new_compound_property:nn { 3 } { \chemformula_subscript:n { 3 } }
\chemformula_new_compound_property:nn { 4 } { \chemformula_subscript:n { 4 } }
\chemformula_new_compound_property:nn { 5 } { \chemformula_subscript:n { 5 } }
\chemformula_new_compound_property:nn { 6 } { \chemformula_subscript:n { 6 } }
\chemformula_new_compound_property:nn { 7 } { \chemformula_subscript:n { 7 } }
\chemformula_new_compound_property:nn { 8 } { \chemformula_subscript:n { 8 } }
\chemformula_new_compound_property:nn { 9 } { \chemformula_subscript:n { 9 } }
\chemformula_new_compound_property:nn { * } { \chemformula_star_adduct: }
\chemformula_new_compound_property:nn { . } { \chemformula_adduct: }
\chemformula_new_compound_property:nn { - } { \chemformula_single_bond: }
\chemformula_new_compound_property:nn { = } { \chemformula_double_bond: }
\chemformula_new_compound_property:nn { + } { \chemformula_triple_bond: }
\chemformula_new_compound_property:nn { ' } { \chemformula_prime: }

% numbers
\prop_put:Nnn \l__chemformula_numbers_prop { 0 } { 0 }
\prop_put:Nnn \l__chemformula_numbers_prop { 1 } { 1 }
\prop_put:Nnn \l__chemformula_numbers_prop { 2 } { 2 }
\prop_put:Nnn \l__chemformula_numbers_prop { 3 } { 3 }
\prop_put:Nnn \l__chemformula_numbers_prop { 4 } { 4 }
\prop_put:Nnn \l__chemformula_numbers_prop { 5 } { 5 }
\prop_put:Nnn \l__chemformula_numbers_prop { 6 } { 6 }
\prop_put:Nnn \l__chemformula_numbers_prop { 7 } { 7 }
\prop_put:Nnn \l__chemformula_numbers_prop { 8 } { 8 }
\prop_put:Nnn \l__chemformula_numbers_prop { 9 } { 9 }

% behaviour in a charge
\prop_put:Nnn \l__chemformula_charge_prop { 0 } { \chemformula_charge_style:n { 0 } }
\prop_put:Nnn \l__chemformula_charge_prop { 1 } { \chemformula_charge_style:n { 1 } }
\prop_put:Nnn \l__chemformula_charge_prop { 2 } { \chemformula_charge_style:n { 2 } }
\prop_put:Nnn \l__chemformula_charge_prop { 3 } { \chemformula_charge_style:n { 3 } }
\prop_put:Nnn \l__chemformula_charge_prop { 4 } { \chemformula_charge_style:n { 4 } }
\prop_put:Nnn \l__chemformula_charge_prop { 5 } { \chemformula_charge_style:n { 5 } }
\prop_put:Nnn \l__chemformula_charge_prop { 6 } { \chemformula_charge_style:n { 6 } }
\prop_put:Nnn \l__chemformula_charge_prop { 7 } { \chemformula_charge_style:n { 7 } }
\prop_put:Nnn \l__chemformula_charge_prop { 8 } { \chemformula_charge_style:n { 8 } }
\prop_put:Nnn \l__chemformula_charge_prop { 9 } { \chemformula_charge_style:n { 9 } }
\prop_put:Nnn \l__chemformula_charge_prop { + } { \chemformula_plus: }
\prop_put:Nnn \l__chemformula_charge_prop { - } { \chemformula_minus: }
\prop_put:Nnn \l__chemformula_charge_prop { . } { \chemformula_radical_or_decimal: }
\prop_put:Nnn \l__chemformula_charge_prop { , } { \chemformula_decimal: }

% stoichiometric factors
\prop_put:Nnn \l__chemformula_stoich_prop { 0 } { 0 }
\prop_put:Nnn \l__chemformula_stoich_prop { 1 } { 1 }
\prop_put:Nnn \l__chemformula_stoich_prop { 2 } { 2 }
\prop_put:Nnn \l__chemformula_stoich_prop { 3 } { 3 }
\prop_put:Nnn \l__chemformula_stoich_prop { 4 } { 4 }
\prop_put:Nnn \l__chemformula_stoich_prop { 5 } { 5 }
\prop_put:Nnn \l__chemformula_stoich_prop { 6 } { 6 }
\prop_put:Nnn \l__chemformula_stoich_prop { 7 } { 7 }
\prop_put:Nnn \l__chemformula_stoich_prop { 8 } { 8 }
\prop_put:Nnn \l__chemformula_stoich_prop { 9 } { 9 }
\prop_put:Nnn \l__chemformula_stoich_prop { . } { \chemformula_decimal: }
\prop_put:Nnn \l__chemformula_stoich_prop { , } { \chemformula_decimal: }
\prop_put:Nnn \l__chemformula_stoich_prop { / } { / }
\prop_put:Nnn \l__chemformula_stoich_prop { _ } { _ }
\prop_put:Nnn \l__chemformula_stoich_prop { ( } { ( }
\prop_put:Nnn \l__chemformula_stoich_prop { ) } { ) }

\prop_put:Nnn \l__chemformula_no_stoich_prop { . } { }
\prop_put:Nnn \l__chemformula_no_stoich_prop { , } { }
\prop_put:Nnn \l__chemformula_no_stoich_prop { / } { }
\prop_put:Nnn \l__chemformula_no_stoich_prop { _ } { }
\prop_put:Nnn \l__chemformula_no_stoich_prop { ( } { }
\prop_put:Nnn \l__chemformula_no_stoich_prop { ) } { }

\prop_put:Nnn \l__chemformula_kroegervink_prop { 0 } { \chemformula_charge_style:n { 0 } }
\prop_put:Nnn \l__chemformula_kroegervink_prop { 1 } { \chemformula_charge_style:n { 1 } }
\prop_put:Nnn \l__chemformula_kroegervink_prop { 2 } { \chemformula_charge_style:n { 2 } }
\prop_put:Nnn \l__chemformula_kroegervink_prop { 3 } { \chemformula_charge_style:n { 3 } }
\prop_put:Nnn \l__chemformula_kroegervink_prop { 4 } { \chemformula_charge_style:n { 4 } }
\prop_put:Nnn \l__chemformula_kroegervink_prop { 5 } { \chemformula_charge_style:n { 5 } }
\prop_put:Nnn \l__chemformula_kroegervink_prop { 6 } { \chemformula_charge_style:n { 6 } }
\prop_put:Nnn \l__chemformula_kroegervink_prop { 7 } { \chemformula_charge_style:n { 7 } }
\prop_put:Nnn \l__chemformula_kroegervink_prop { 8 } { \chemformula_charge_style:n { 8 } }
\prop_put:Nnn \l__chemformula_kroegervink_prop { 9 } { \chemformula_charge_style:n { 9 } }
\prop_put:Nnn \l__chemformula_kroegervink_prop { + } { \chemformula_plus: }
\prop_put:Nnn \l__chemformula_kroegervink_prop { - } { \chemformula_minus: }
\prop_put:Nnn \l__chemformula_kroegervink_prop { . } { \chemformula_radical_or_decimal: }
\prop_put:Nnn \l__chemformula_kroegervink_prop { , } { \chemformula_decimal: }
\prop_put:Nnn \l__chemformula_kroegervink_prop { ' } { \chemformula_kroegervink_negative: }
\prop_put:Nnn \l__chemformula_kroegervink_prop { x } { \chemformula_kroegervink_neutral: }
\prop_put:Nnn \l__chemformula_kroegervink_prop { * } { \chemformula_kroegervink_positive: }
\prop_put:Nnn \l__chemformula_kroegervink_prop { . } { \chemformula_kroegervink_positive: }

% --------------------------------------------------------------------------
% bonds:
\cs_new_protected:Npn \chemformula@bondlength
  { \l__chemformula_bond_dim }
  
\cs_new_protected:Npn \chemformula_single_bond:
  { \chemformula_bond:n {single} }

\cs_new_protected:Npn \chemformula_double_bond:
  { \chemformula_bond:n {double} }

\cs_new_protected:Npn \chemformula_triple_bond:
  { \chemformula_bond:n {triple} }

\cs_new_protected:Npn \chemformula_bond:n #1
  {
    \chemformula_skip_nobreak:N \l__chemformula_bond_space_dim
    \hbox_set:Nn \l__chemformula_tmpa_box {K}
    \dim_set:Nn \l__chemformula_tmpa_dim { \box_ht:N \l__chemformula_tmpa_box }
    \chemformula_tikz:nn
      {
        inner~sep   = 0pt ,
        outer~sep   = 0pt ,
        text~height = \l__chemformula_tmpa_dim ,
        baseline    = (chemformula-bond-ground.base)
      }
      {
        \coordinate (chemformula-bond-ground) at (0pt,0pt) ;
        \draw (chemformula-bond-ground) ++ (0pt,.5\l__chemformula_tmpa_dim)
          coordinate (chemformula-bond-start) ;
        \draw (chemformula-bond-start) ++(\l__chemformula_bond_dim ,0pt)
          coordinate (chemformula-bond-end) ;
        \tl_if_blank:nTF {#1}
          { \__chemformula_bond_draw:n {single} }
          { \__chemformula_bond_draw:n {#1} }
      }
    \chemformula_skip_nobreak_penalty:NV
      \l__chemformula_bond_space_dim
      \l__chemformula_bond_penalty_tl
  }

\cs_new_protected:Npn \__chemformula_bond_draw:n #1
  {
    \__chemformula_reset:Nn \l__chemformula_tmpa_tl {#1}
    \prop_item:NV \l__chemformula_bonds_prop
      \l__chemformula_tmpa_tl
  }

\cs_new_protected:Npn \chemformula_declare_bond:nn #1#2
  {
    \cs_if_exist:cTF { chemformula_bond_type_#1: }
      { \cs_set:cpn { chemformula_bond_type_#1: } {} }
      { \cs_new_protected:cpn { chemformula_bond_type_#1: } {} }
    \prop_put:Nnn \l__chemformula_bonds_prop {#1} {#2}
  }

\cs_new_protected:Npn \chemformula_new_bond:nn #1#2
  {
    \cs_if_exist:cTF { chemformula_bond_type_#1: }
      { \msg_error:nnn {chemformula} {new-bond} {#1} }
      { \chemformula_declare_bond:nn {#1} {#2} }
  }

\cs_new_protected:Npn \chemformula_provide_bond:nn #1#2
  {
    \cs_if_exist:cF { chemformula_bond_type_#1: }
      { \chemformula_declare_bond:nn {#1} {#2} }
  }

\cs_new_protected:Npn \chemformula_renew_bond:nn #1#2
  {
    \cs_if_exist:cTF { chemformula_bond_type_#1: }
      { \chemformula_declare_bond:nn {#1} {#2} }
      { \msg_error:nnn {chemformula} {renew-bond} {#1} }
  }

\cs_new_protected:Npn \chemformula_new_bond_alias:nn #1#2
  {
    \cs_if_exist:cTF { chemformula_bond_type_#2: }
      {
        \cs_if_exist:cTF { chemformula_bond_type_#1: }
          { \msg_error:nnn {chemformula} {new-bond} {#1} }
          { \cs_new_protected:cpn { chemformula_bond_type_#1: } {} }
        \prop_get:NnN \l__chemformula_bonds_prop {#2} \l__chemformula_tmpa_tl
        \prop_put:Nno \l__chemformula_bonds_prop {#1} { \l__chemformula_tmpa_tl }
      }
      { \msg_error:nnnn {chemformula} {bond-alias} {#1} {#2} }
  }

\cs_new_protected:Npn \chemformula_declare_bond_alias:nn #1#2
  {
    \cs_if_exist:cTF { chemformula_bond_type_#2: }
      {
        \cs_if_exist:cF { chemformula_bond_type_#1: }
          { \cs_new_protected:cpn { chemformula_bond_type_#1: } {} }
        \prop_get:NnN \l__chemformula_bonds_prop {#2} \l__chemformula_tmpa_tl
        \prop_put:Nno \l__chemformula_bonds_prop {#1} { \l__chemformula_tmpa_tl }
      }
      { \msg_error:nnnn {chemformula} {bond-alias} {#1} {#2} }
  }

\cs_new_protected:Npn \chemformula_show_bond:n #1
  {
    \prop_get:NnN \l__chemformula_bonds_prop {#1} \l__chemformula_tmpa_tl
    \tl_to_str:N \l__chemformula_tmpa_tl
  }

\NewDocumentCommand \NewChemBond {mm}
  { \chemformula_new_bond:nn {#1} {#2} }

\NewDocumentCommand \ProvideChemBond {mm}
  { \chemformula_provide_bond:nn {#1} {#2} }
  
\NewDocumentCommand \DeclareChemBond {mm}
  { \chemformula_declare_bond:nn {#1} {#2} }

\NewDocumentCommand \RenewChemBond {mm}
  { \chemformula_renew_bond:nn {#1} {#2} }

\NewDocumentCommand \NewChemBondAlias {mm}
  { \chemformula_new_bond_alias:nn {#1} {#2} }

\NewDocumentCommand \DeclareChemBondAlias {mm}
  { \chemformula_declare_bond_alias:nn {#1} {#2} }

\NewDocumentCommand \ShowChemBond {m}
  { \chemformula_show_bond:n {#1} }

\NewDocumentCommand \bond {m}
  { \chemformula_bond:n {#1} }

% now let's declare some default bonds:
\NewChemBond {single}
  { \draw[chembond] (chemformula-bond-start) -- (chemformula-bond-end) ; }
\NewChemBondAlias {normal} {single}
\NewChemBondAlias {sb}     {single}

\NewChemBond {semisingle}
  { \draw[chembond,densely~dotted] (chemformula-bond-start) -- (chemformula-bond-end) ; }
\NewChemBondAlias {dotted} {semisingle}

\NewChemBond {double}
  {
    \foreach \i in {-.0858em,.0858em}{
      \draw[chembond]
        ([yshift=\i]chemformula-bond-start) -- ([yshift=\i]chemformula-bond-end) ;
    }
  }
\NewChemBondAlias {db} {double}

\NewChemBond {semidouble}
  {
    \foreach \i/\style in {-.0858em/,.0858em/densely~dotted}{
      \draw[chembond,\style]
        ([yshift=\i]chemformula-bond-start) -- ([yshift=\i]chemformula-bond-end) ;
    }
  }

\NewChemBondAlias {deloc} {semidouble}

\NewChemBond {triple}
  {
    \foreach \i in {-.1287em,0em,.1287em}{
      \draw[chembond]
        ([yshift=\i]chemformula-bond-start) -- ([yshift=\i]chemformula-bond-end) ;
    }
  }
\NewChemBondAlias {tp} {triple}

\NewChemBond {semitriple}
  {
    \foreach \i/\style in {-.1287em/,0em/,.1287em/densely~dotted}{
      \draw[chembond,\style]
        ([yshift=\i]chemformula-bond-start) -- ([yshift=\i]chemformula-bond-end) ;
    }
  }
\NewChemBondAlias {tdeloc} {semitriple}

\NewChemBond {coordright}
  { \draw[chembond,Butt~Cap->] (chemformula-bond-start) -- (chemformula-bond-end) ; }
\NewChemBondAlias {co>} {coordright}

\NewChemBond {coordleft}
  { \draw[chembond,<-Butt~Cap] (chemformula-bond-start) -- (chemformula-bond-end) ; }
\NewChemBondAlias {<co} {coordleft}

% --------------------------------------------------------------------------
% sub- and superscripts
\cs_new_protected:Npn \__chemformula_number_style:n #1
  {
    \bool_if:NTF \l__chemformula_number_style_math_bool
      { \ensuremath {#1} }
      { \ensuremath { \text {#1} } }
  }
\cs_generate_variant:Nn \__chemformula_number_style:n {V}

\cs_new_protected:Npn \chemformula_charge_style:n #1
  {
    \bool_if:NTF \l__chemformula_charge_style_math_bool
      { \ensuremath {#1} }
      { \ensuremath { \text {#1} } }
  }
\cs_generate_variant:Nn \chemformula_charge_style:n {V}

% subscripts
% remove double or nested sub- and superscripts
\cs_new_protected:Npn \chemformula_subscript:n #1
  {
    \chemformula_nobreak:
    \tl_if_eq:nnTF {#1} { \chemformula_superscript:n }
      { \__chemformula_subscript:n }
      {
        \tl_if_eq:nnTF {#1} { \chemformula_subscript:n }
          { \__chemformula_subscript:n }
          {
            \tl_set:Nn \l__chemformula_tmpc_tl {#1}
            \tl_if_in:nnT {#1} { \chemformula_subscript:n }
              { \tl_remove_all:Nn \l__chemformula_tmpc_tl { \chemformula_subscript:n } }
            \tl_if_in:nnT {#1} { \chemformula_superscript:n }
              { \tl_remove_all:Nn \l__chemformula_tmpc_tl { \chemformula_superscript:n } }
            \__chemformula_subscript:V \l__chemformula_tmpc_tl
          }
      }
  }

% handle subscripts
\cs_new_protected:Npn \__chemformula_subscript:n #1
  {
    \tl_put_right:Nn \l__chemformula_subscript_tl {#1}
    \__chemformula_clean_subscript:
    % if a subscript follows just store and do nothing else
    \peek_meaning:NF \chemformula_subscript:n
      {
        % if a superscript follows store and do nothing else
        % if something else follows both sub- and superscript
        \peek_meaning:NF \chemformula_superscript:n
          {
            \__chemformula_measure_subscript:V \l__chemformula_subscript_tl
            \__chemformula_measure_superscript:V \l__chemformula_superscript_tl
            \__chemformula_use_sub_and_superscript:
          }
      }
  }
\cs_generate_variant:Nn \__chemformula_subscript:n {V}

% TODO: add hook for cleanup?
% similar to ChemCompoundProperty?
\prop_new:N \l__chemformula_clean_subscript_prop
\seq_new:N \l__chemformula_clean_subscript_seq

\cs_new_protected:Npn \chemformula_add_subscript_cleanup:nn #1#2
  {
    \seq_put_right:Nn \l__chemformula_clean_subscript_seq {#1}
    \prop_put:Nnn \l__chemformula_clean_subscript_prop {#1} {#2}
  }

% \NewDocumentCommand \ChemCleanSubscript {mm}
%   { \chemformula_add_subscript_cleanup:nn {#1} {#2} }

\chemformula_add_subscript_cleanup:nn
  { \chemformula_single_bond: }
  { \chemformula_minus: }
\chemformula_add_subscript_cleanup:nn
  { \chemformula_triple_bond: }
  { \chemformula_plus: }
\chemformula_add_subscript_cleanup:nn
  { \chemformula_adduct: }
  {.}
\chemformula_add_subscript_cleanup:nn
  { \chemformula_star_adduct: }
  {*}

\cs_new_protected:Npn \__chemformula_clean_subscript:
  {
    \seq_map_inline:Nn \l__chemformula_clean_subscript_seq
      {
        \tl_replace_all:Nnx \l__chemformula_subscript_tl
          {##1}
          { \prop_item:Nn \l__chemformula_clean_subscript_prop {##1} }
      }
  }
\cs_generate_variant:Nn \tl_replace_all:Nnn {Nnx}

\cs_new_protected:Npn \__chemformula_subscript_raise:N #1
  {
    \hbox_set:Nn \l__chemformula_tmpa_box
      { \c_math_toggle_token \scriptstyle Hg \c_math_toggle_token }
    \hbox_set:Nn \l__chemformula_tmpb_box { Hg }
    \dim_set:Nn #1
      {
        - \box_ht:N \l__chemformula_tmpa_box
        + .4 \box_ht:N \l__chemformula_tmpb_box
        + \l__chemformula_subscript_shift_additional_tl
      }
    \chemformula_leave_vmode:
    \tex_raise:D #1
  }

\cs_new_protected:Npn \__chemformula_subscript_write:n #1
  {
    \chemformula_nobreak:
    \group_begin:
      \bool_set_true:N \l__chemformula_inside_subscript_bool
      \tl_set_eq:NN \chemformula_subscript:n \use:n
      \tl_set_eq:NN \chemformula_superscript:n \use:n
      \__chemformula_subscript_raise:N \l__chemformula_subscript_shift_dim
      \hbox:n
        {
          \c_math_toggle_token
            \scriptstyle
            \prop_get:NoNTF \l__chemformula_numbers_prop
              {#1} \l__chemformula_tmpa_tl
              { \__chemformula_number_style:V \l__chemformula_tmpa_tl }
              { \__chemformula_number_style:n {#1} }
          \c_math_toggle_token
        }
    \group_end:
  }
\cs_generate_variant:Nn \__chemformula_subscript_write:n {V}

\cs_new_protected:Npn \__chemformula_measure_subscript:n #1
  {
    \hbox_set:Nn \l__chemformula_tmpa_box
      { \__chemformula_subscript_write:n {#1} }
    \dim_set:Nn \l__chemformula_subscript_dim
      { -\box_wd:N \l__chemformula_tmpa_box }
    \box_clear:N \l__chemformula_tmpa_box
  }
\cs_generate_variant:Nn \__chemformula_measure_subscript:n {V}

% superscripts
% remove double or nested sub- and superscripts
\cs_new_protected:Npn \chemformula_superscript:n #1
  {
    \chemformula_nobreak:
    \tl_if_eq:nnTF {#1} { \chemformula_superscript:n }
      { \__chemformula_superscript:n }
      {
        \tl_if_eq:nnTF {#1} { \chemformula_subscript:n }
          { \__chemformula_superscript:n }
          {
            \tl_set:Nn \l__chemformula_tmpc_tl {#1}
            \tl_if_in:nnT {#1} { \chemformula_subscript:n }
              { \tl_remove_all:Nn \l__chemformula_tmpc_tl { \chemformula_subscript:n } }
            \tl_if_in:nnT {#1} { \chemformula_superscript:n }
              { \tl_remove_all:Nn \l__chemformula_tmpc_tl { \chemformula_superscript:n } }
            \__chemformula_superscript:V \l__chemformula_tmpc_tl
          }
      }
  }

% handle superscripts
\cs_new_protected:Npn \__chemformula_superscript:n #1
  {
    \tl_put_right:Nn \l__chemformula_superscript_tl {#1}
    \__chemformula_clean_superscript:
    \peek_meaning:NF \chemformula_superscript:n
      {
        \peek_meaning:NF \chemformula_subscript:n
          {
            \__chemformula_measure_subscript:V \l__chemformula_subscript_tl
            \__chemformula_measure_superscript:V \l__chemformula_superscript_tl
            \__chemformula_use_sub_and_superscript:
          }
      }
  }
\cs_generate_variant:Nn \__chemformula_superscript:n {V}

% TODO: add hook for cleanup?
% \prop_new:N \l__chemformula_clean_superscript_prop
% \seq_new:N \l__chemformula_clean_superscript_seq

% \cs_new_protected:Npn \chemformula_add_superscript_cleanup:nnn #1#2#3
%   {
%     \seq_put_right:Nn \l__chemformula_clean_superscript_seq {#1}
%     \prop_put:Nnn \l__chemformula_clean_superscript_prop {#1}
%       { \bool_if:NTF \l__chemformula_kroegervink_bool {#2} {#3} }
%   }

% \chemformula_add_superscript_cleanup:nnn
%   { \chemformula_single_bond: }
%   { \chemformula_minus: }
%   { \chemformula_minus: }
% \chemformula_add_superscript_cleanup:nnn
%   { - }
%   { \chemformula_minus: }
%   { \chemformula_minus: }
% \chemformula_add_superscript_cleanup:nnn
%   { \chemformula_triple_bond: }
%   { \chemformula_plus: }
%   { \chemformula_plus: }
% \chemformula_add_superscript_cleanup:nnn
%   { + }
%   { \chemformula_plus: }
%   { \chemformula_plus: }
% \chemformula_add_superscript_cleanup:nnn
%   { . }
%   { \chemformula_radical_or_decimal: }
%   { \chemformula_kroegervink_positive: }
% \chemformula_add_superscript_cleanup:nnn
%   { \chemformula_adduct: }
%   { \chemformula_radical_or_decimal: }
%   { \chemformula_kroegervink_positive: }
% \chemformula_add_superscript_cleanup:nnn
%   { \chemformula_radical_or_decimal: }
%   { \chemformula_radical_or_decimal: }
%   { \chemformula_kroegervink_positive: }
% \chemformula_add_superscript_cleanup:nnn
%   { \chemformula_star_adduct: }
%   { * }
%   { \chemformula_kroegervink_positive: }
% \chemformula_add_superscript_cleanup:nnn
%   { * }
%   { * }
%   { \chemformula_kroegervink_positive: }
% \chemformula_add_superscript_cleanup:nnn
%   { ' }
%   { ' }
%   { \chemformula_kroegervink_negative: }
% \chemformula_add_superscript_cleanup:nnn
%   { \chemformula_prime: }
%   { \chemformula_prime: }
%   { \chemformula_kroegervink_negative: }
% \chemformula_add_superscript_cleanup:nnn
%   { , }
%   { \chemformula_decimal: }
%   { , }

% rename this function into treatment instead of cleanup
\cs_new_protected:Npn \__chemformula_clean_superscript:
  {
    % \seq_map_inline:Nn \l__chemformula_clean_superscript_seq
    %   {
    %     \tl_replace_all:Nnx \l__chemformula_superscript_tl
    %       {##1}
    %       { \prop_item:Nn \l__chemformula_clean_superscript_prop {##1} }
    %   }
    \tl_replace_all:Nnn \l__chemformula_superscript_tl
      { \chemformula_single_bond: } { \chemformula_minus: }
    \tl_replace_all:Nnn \l__chemformula_superscript_tl
      {-} { \chemformula_minus: }
    \tl_replace_all:Nnn \l__chemformula_superscript_tl
      { \chemformula_triple_bond: } { \chemformula_plus: }
    \tl_replace_all:Nnn \l__chemformula_superscript_tl
      {+} { \chemformula_plus: }
    \tl_replace_all:Nnn \l__chemformula_superscript_tl
      { \chemformula_adduct: } { \chemformula_radical_or_decimal: }
    \bool_if:NTF \l__chemformula_kroegervink_bool
      {
        \tl_replace_all:Nnn \l__chemformula_superscript_tl
          {'} { \chemformula_kroegervink_negative: }
        \tl_replace_all:Nnn \l__chemformula_superscript_tl
          { \chemformula_adduct: } {.}
        \tl_replace_all:Nnn \l__chemformula_superscript_tl
          { \chemformula_star_adduct: } {*}
        \tl_replace_all:Nnn \l__chemformula_superscript_tl
          { \chemformula_radical_or_decimal: } {.}
        \tl_replace_all:Nnn \l__chemformula_superscript_tl
          { \chemformula_prime: } { \chemformula_kroegervink_negative: }
        \tl_replace_all:Nnn \l__chemformula_superscript_tl
          {*} { \chemformula_kroegervink_positive: }
        \tl_replace_all:Nnn \l__chemformula_superscript_tl
          {.} { \chemformula_kroegervink_positive: }
      }
      {
        \tl_replace_all:Nnn \l__chemformula_superscript_tl
          { \chemformula_star_adduct: } {*}
        \tl_replace_all:Nnn \l__chemformula_superscript_tl
          {.} { \chemformula_radical_or_decimal: }
        \tl_replace_all:Nnn \l__chemformula_superscript_tl
          {,} { \chemformula_decimal: }
      }
  }

\cs_new_protected:Npn \__chemformula_superscript_raise:N #1
  {
    \hbox_set:Nn \l__chemformula_tmpa_box
      { \c_math_toggle_token \scriptstyle Hg \c_math_toggle_token }
    \hbox_set:Nn \l__chemformula_tmpb_box { Hg }
    \dim_set:Nn #1
      {
        + \box_ht:N \l__chemformula_tmpb_box
        - .5 \box_ht:N \l__chemformula_tmpa_box
        + \l__chemformula_superscript_shift_additional_tl
      }
    \chemformula_leave_vmode:
    \tex_raise:D #1
  }

\cs_new_protected:Npn \__chemformula_superscript_write:n #1
  {
    \chemformula_nobreak:
    \group_begin:
      \bool_set_true:N \l__chemformula_inside_superscript_bool
      \tl_set_eq:NN \chemformula_subscript:n \use:n
      \tl_set_eq:NN \chemformula_superscript:n \use:n
      \__chemformula_superscript_raise:N \l__chemformula_superscript_shift_dim
      \hbox:n
        {
          \c_math_toggle_token
            \scriptstyle
            % TODO: this is at the wrong place here -- we need to map through
            % the superscript tokens!
            \bool_if:NTF \l__chemformula_kroegervink_bool
              { % TODO: is the o argument necessary?
                \prop_get:NoNTF \l__chemformula_kroegervink_prop
                  {#1} \l__chemformula_tmpa_tl
                  { \tl_use:N \l__chemformula_tmpa_tl }
                  { \chemformula_charge_style:n {#1} }
              }
              { % TODO: is the o argument necessary?
                \prop_get:NoNTF \l__chemformula_charge_prop
                  {#1} \l__chemformula_tmpa_tl
                  { \tl_use:N \l__chemformula_tmpa_tl }
                  { \chemformula_charge_style:n {#1} }
              }
          \c_math_toggle_token
        }
    \group_end:
  }
\cs_generate_variant:Nn \__chemformula_superscript_write:n {V}

\cs_new_protected:Npn \__chemformula_measure_superscript:n #1
  {
    \hbox_set:Nn \l__chemformula_tmpa_box
      { \__chemformula_superscript_write:n {#1} }
    \dim_set:Nn \l__chemformula_superscript_dim
      { -\box_wd:N \l__chemformula_tmpa_box }
    \box_clear:N \l__chemformula_tmpa_box
  }
\cs_generate_variant:Nn \__chemformula_measure_superscript:n {V}

% typeset both sub- and superscripts
\cs_new_protected:Npn \__chemformula_use_sub_and_superscript:
  {
    \group_begin:
    % No shift for excited state
    \tl_if_eq:VnT \l__chemformula_superscript_tl {*}
      {
        \bool_set_false:N \l__chemformula_charge_full_shift_bool
        \dim_zero:N \l__chemformula_charge_shift_dim
      }
    % no shift for Kroeger Vink notation
    \bool_if:NT \l__chemformula_kroegervink_bool
      {
        \bool_set_false:N \l__chemformula_charge_full_shift_bool
        \dim_zero:N \l__chemformula_charge_shift_dim
      }
    % align to the right if isotope else to the left
    \bool_if:NTF \l__chemformula_is_isotope_bool
      {
        \tl_if_blank:VF \l__chemformula_superscript_tl
          {
            \dim_compare:nT
              { \l__chemformula_subscript_dim < \l__chemformula_superscript_dim }
              {
                \chemformula_skip_nobreak:n { -\l__chemformula_subscript_dim }
                \chemformula_skip_nobreak:N \l__chemformula_superscript_dim
              }
          }
      }
      {
        % add hshift to the superscript if there also is a subscript
        \tl_if_blank:VF \l__chemformula_superscript_tl
          {
            \tl_if_blank:VF \l__chemformula_subscript_tl
              {
                \bool_if:NTF \l__chemformula_charge_full_shift_bool
                  { \chemformula_skip_nobreak:n { -\l__chemformula_subscript_dim } }
                  { \chemformula_skip_nobreak:N \l__chemformula_charge_shift_dim }
              }
          }
      }
    % typeset superscript
    \tl_if_blank:VF \l__chemformula_superscript_tl
      {
       \__chemformula_superscript_write:V \l__chemformula_superscript_tl
        % skip back for the subscript
        \tl_if_blank:VF \l__chemformula_subscript_tl
          {
            \bool_if:NTF \l__chemformula_is_isotope_bool
              { \chemformula_skip_nobreak:N \l__chemformula_subscript_dim }
              {
                \chemformula_skip_nobreak:N \l__chemformula_superscript_dim
                \bool_if:NTF \l__chemformula_charge_full_shift_bool
                  { \chemformula_skip_nobreak:N \l__chemformula_subscript_dim }
                  {
                    \chemformula_skip_nobreak:n
                      { -\l__chemformula_charge_shift_dim }
                  }
              }
          }
      }
    % typeset subscript
    \tl_if_blank:VF \l__chemformula_subscript_tl
      { \__chemformula_subscript_write:V \l__chemformula_subscript_tl }
    \bool_if:NF \l__chemformula_is_isotope_bool
      {
        \dim_zero:N \l__chemformula_tmpa_dim
        \bool_if:NTF \l__chemformula_charge_full_shift_bool
          {
            \tl_if_blank:VF \l__chemformula_subscript_tl
              { \dim_set:Nn \l__chemformula_tmpa_dim
                { - \l__chemformula_superscript_dim } }
          }
          {
            \tl_if_blank:VF \l__chemformula_superscript_tl
              {
                \tl_if_blank:VF \l__chemformula_subscript_tl
                  {
                    \dim_set:Nn \l__chemformula_tmpa_dim
                      {
                        \l__chemformula_charge_shift_dim
                        - \l__chemformula_superscript_dim
                        + \l__chemformula_subscript_dim
                      }
                  }
              }
          }
        \dim_compare:nT { \l__chemformula_tmpa_dim > 0pt }
          { \chemformula_skip_nobreak:N \l__chemformula_tmpa_dim }
      }
    \group_end:
    % clean up
    \bool_set_false:N \l__chemformula_is_isotope_bool
    \tl_clear:N \l__chemformula_subscript_tl
    \tl_clear:N \l__chemformula_superscript_tl
  }

% --------------------------------------------------------------------------
% read the input
\cs_new_protected:Npn \__chemformula_input:n #1
  {
    \__chemformula_save_catcodes:
    \__chemformula_sanitize:Nn \l__chemformula_chemformula_tmpa_tl {#1}
    \__chemformula_input_escape_text:V \l__chemformula_chemformula_tmpa_tl
    \__chemformula_input_escape_math:n {#1}
    \__chemformula_input_stoich:n {#1}
    \__chemformula_clean_chemmacros:V \l__chemformula_chemformula_tmpa_tl
    \__chemformula_input_compound:NV
      \l__chemformula_input_tl
      \l__chemformula_chemformula_tmpa_tl
    \__chemformula_input_arrow:n {#1}
    \__chemformula_input_name:n {#1}
    \__chemformula_input_addition:n {#1}
    \__chemformula_input_symbol:n {#1}
  }

% --------------------------------------------------------------------------
% set options for next compound:
\cs_new_protected:Npn \__chemformula_input_options:n #1
  {
    % \makeatother
    \char_set_catcode_other:N \@
    \tl_set_rescan:Nnn \l__chemformula_tmpa_tl
      { \char_set_catcode_letter:N \@ }
      {#1}
    \tl_if_in:VnTF \l__chemformula_tmpa_tl { @ }
      {
        \bool_set_false:N \l__chemformula_options_bool
        \bool_set_true:N \l__chemformula_is_option_bool
        \tl_remove_all:Nn \l__chemformula_tmpa_tl { @ }
        \exp_last_unbraced:Nno
        \tl_set:Nn \l__chemformula_internal_options_tl
          { \l__chemformula_tmpa_tl }
      }
      { \bool_set_false:N \l__chemformula_is_option_bool }
  }

% --------------------------------------------------------------------------
% input stoichiometric factors
\cs_new_protected:Npn \chemformula_decimal: { { \l__chemformula_decimal_output_tl } }

\cs_new_protected:Npn \__chemformula_input_stoich:n #1
  {
    \bool_set_true:N \l__chemformula_stoich_bool
    % the factor consists only of a parenthesis or a decimal marcer:
    \bool_set_true:N  \l__chemformula_no_stoich_bool
    \tl_map_inline:nn {#1}
      {
        \prop_if_in:NnF \l__chemformula_no_stoich_prop {##1}
          { \bool_set_false:N \l__chemformula_no_stoich_bool }
      }
    \bool_if:NF \l__chemformula_no_stoich_bool
      {
        \tl_map_inline:nn {#1}
          {
            \prop_get:NnNTF \l__chemformula_stoich_prop {##1} \l__chemformula_tmpa_tl
              {
                \bool_if:NT \l__chemformula_stoich_bool
                  { \tl_put_right:NV \l__chemformula_stoich_tl \l__chemformula_tmpa_tl }
              }
              { \bool_set_false:N \l__chemformula_stoich_bool }
           }
      }
    \bool_if:NTF \l__chemformula_stoich_bool
      {
        \tl_if_blank:VF \l__chemformula_stoich_tl
          {
            \tl_put_left:Nn \l__chemformula_stoich_tl { \q_mark }
            \__chemformula_sanitize:NV
              \l__chemformula_stoich_tl
              \l__chemformula_stoich_tl
            \__chemformula_print_stoich:V
              \l__chemformula_stoich_tl
          }
      }
      { \tl_clear:N \l__chemformula_stoich_tl }
  }

\cs_new_protected:Npn \chemformula_bm:n #1 {#1}

\cs_new_protected:Npn \__chemformula_print_stoich:n #1
  { \chemformula_parse_stoich:n {#1} }
\cs_generate_variant:Nn \__chemformula_print_stoich:n {V}

\cs_new_protected:Npn \chemformula_parse_stoich:n #1
  { \__chemformula_parse_stoich:w #1 \q_nil }

\NewDocumentCommand \chstoich {m}
  { \chemformula_parse_stoich:n {#1} }

\cs_new_protected:Npn \__chemformula_parse_stoich:w \q_mark #1 \q_nil
  {
    \__chemformula_bool_set_if_first_last:Nnnn
      \l__chemformula_stoich_is_iupac_bool {#1} { ( } { ) }
    \bool_if:NTF \l__chemformula_stoich_is_iupac_bool
      { \__chemformula_parse_stoich_iupac:n {#1} }
      { \__chemformula_parse_stoich_regular:n {#1} }
  }

\DeclareInstance { xfrac } { chemformula-text-frac } { text }
  {
    slash-left-kern     = -.15em ,
    numerator-top-sep   = 0pt ,
    slash-right-kern    = -.15em
  }

\cs_new_protected:Npn \__chemformula_parse_stoich_regular:n #1
  {
    \tl_if_in:nnTF {#1} { / }
      { \__chemformula_parse_stoich_frac:n {#1} }
      { \__chemformula_parse_stoich_decimal:n {#1} }
  }

\cs_new_protected:Npn \__chemformula_parse_stoich_decimal:n #1
  {
    % get head:
    \tl_set:Nx \l__chemformula_tmpa_tl { \tl_head:n {#1} }
    \tl_set:Nn \l__chemformula_tmpb_tl { \chemformula_decimal: }
    % check if stoich starts with decimal-marker and add leading 0 if so:
    \tl_if_eq:NNTF \l__chemformula_tmpa_tl \l__chemformula_tmpb_tl
      { \tl_put_right:Nn \l__chemformula_input_tl { 0#1 } }
      { \tl_put_right:Nn \l__chemformula_input_tl {#1} }
  }

\cs_new_protected:Npn \__chemformula_parse_stoich_frac:n #1
  {
    \tl_if_in:nnTF {#1} { _ }
      { \__chemformula_parse_stoich_misc_frac:w #1 \q_nil }
      { \__chemformula_parse_stoich_frac:w #1 \q_nil }
  }

\cs_new_protected:Npn \__chemformula_parse_stoich_frac:w #1/#2 \q_nil
  {
    \tl_put_right:Nn \l__chemformula_input_tl
      { \__chemformula_frac:nn {#1} {#2} }
  }

\cs_new_protected:Npn \__chemformula_parse_stoich_misc_frac:w #1_#2/#3 \q_nil
  {
    \tl_put_right:Nn \l__chemformula_input_tl
      { \__chemformula_misc_frac:nnn {#1} {#2} {#3} }
  }

\cs_new_protected:Npn \__chemformula_parse_stoich_iupac:n #1
  {
    \bool_if:NTF \l__chemformula_stoich_parse_iupac_bool
      { \__chemformula_parse_stoich_iupac:w #1 \q_nil }
      { \tl_put_right:Nn \l__chemformula_input_tl {#1} }
  }

\cs_new_protected:Npn \__chemformula_parse_stoich_iupac:w (#1) \q_nil
  {
    \tl_put_right:Nn \l__chemformula_input_tl { ( }
    \__chemformula_parse_stoich_regular:n {#1}
    \tl_put_right:Nn \l__chemformula_input_tl { ) }
  }

\cs_new_eq:NN \chemformula_math_frac:nn \frac

\cs_new_protected:Npn \__chemformula_frac:nn #1#2
  {
    \bool_if:NT \l__chemformula_xfrac_bool
      { \sfrac[chemformula-text-frac]{#1}{#2} }
    \bool_if:NT \l__chemformula_nicefrac_bool
      { \nicefrac{#1}{#2} }
    \bool_if:NT \l__chemformula_mathfrac_bool
      {
        \c_math_toggle_token
          \chemformula_math_frac:nn { \text {#1} } { \text {#2} }
        \c_math_toggle_token
      }
  }

\cs_new_protected:Npn \__chemformula_misc_frac:nnn #1#2#3
  {
    \bool_if:NT \l__chemformula_xfrac_bool
      { #1\sfrac[chemformula-text-frac]{#2}{#3} }
    \bool_if:NT \l__chemformula_nicefrac_bool
      { #1\nicefrac{#2}{#3} }
    \bool_if:NT \l__chemformula_mathfrac_bool
      {
        #1
        \c_math_toggle_token
          \chemformula_math_frac:nn { \text {#2} } { \text {#3} }
        \c_math_toggle_token
      }
  }

% --------------------------------------------------------------------------
% input compounds
\cs_new_protected:Npn \__chemformula_input_compound:Nn #1#2
  {
    % \tl_show:n {#1}
    \tl_if_blank:VTF \l__chemformula_stoich_tl
      {
        % TODO
        \__chemformula_detect_addition_symbols:n {#2}
        \__chemformula_detect_symbols:n {#2}
        \__chemformula_detect_arrows:n {#2}
        \__chemformula_detect_name:n {#2}
        \__chemformula_input_options:n {#2}
        \bool_if:NT \l__chemformula_is_option_bool
          {
            \bool_set_true:N \l__chemformula_options_bool
            \tl_put_right:Nx #1
              {
                \group_begin:
                  % open group
                  \keys_set:nn
                    {chemformula}
                    { \exp_not:V \l__chemformula_internal_options_tl }
              }
          }
        \bool_if:nT
          {
            !\l__chemformula_is_option_bool &&
            !\l__chemformula_is_addition_symbol_bool &&
            !\l__chemformula_is_symbol_bool &&
            !\l__chemformula_is_arrow_bool &&
            !\l__chemformula_is_name_bool &&
            !\l__chemformula_first_last_text_bool &&
            !\l__chemformula_first_last_math_bool
          }
          {
            \__chemformula_input_compound_no_check:Nn #1 {#2}
            \bool_if:NT \l__chemformula_options_bool
              {
                \tl_put_right:Nn #1
                  {
                    % end group
                    \group_end:
                    \tl_clear:N \l__chemformula_internal_options_tl
                  }
                \bool_set_false:N \l__chemformula_options_bool
              }
          }
      }
      {
        \tl_put_right:Nn #1
          { \chemformula_skip_nobreak:N \l__chemformula_stoich_space_skip }
        \tl_clear:N \l__chemformula_stoich_tl
      }
  }
\cs_generate_variant:Nn \__chemformula_input_compound:Nn {NV}

% TODO: this is redundant at the moment
\bool_new:N \g__chemformula_compound_group_open_bool
\cs_new_protected:Npn \chemformula_group_begin:
  {
    \bool_if:NF \g__chemformula_compound_group_open_bool
      {
        % \group_begin:
        \bool_gset_true:N \g__chemformula_compound_group_open_bool
      }
  }
\cs_new_protected:Npn \chemformula_group_end:
  {
    \bool_if:NT \g__chemformula_compound_group_open_bool
      {
        % \group_end:
        \bool_gset_false:N \g__chemformula_compound_group_open_bool
      }
  }

\cs_new_protected:Npn \chemformula_font_inner:
  {
    \bool_if:NT \l__chemformula_fss_bool
      {
        \bool_if:NT \l__chemformula_fss_family_bool
          { \fontfamily { \l__chemformula_font_family_tl } }
        \bool_if:NT \l__chemformula_fss_series_bool
          { \fontseries { \l__chemformula_font_series_tl } }
        \bool_if:NT \l__chemformula_fss_shape_bool
          { \fontshape { \l__chemformula_font_shape_tl } }
        \selectfont
      }
    \bool_if:NT \l__chemformula_fontspec_bool
      { \cs_if_exist_use:N \chemformula_font: }
    \tl_use:N \l__chemformula_format_tl
    \tl_if_eq:onTF { \f@series } {bx}
      { \cs_set_eq:NN \chemformula_bm:n \bm }
      { \cs_set:Nn \chemformula_bm:n {##1} }
  }

% \cs_new_protected:Npn \chemformula_font: {}
\cs_new_protected:Npn \__chemformula_fontspec:n #1
  {
    \tl_if_in:nnTF {#1} {[}
      { \__chemformula_fontspec_aux:w #1 \q_stop }
      { \__chemformula_fontspec_aux:w [] #1 \q_stop }
  }

\cs_new_protected:Npn \__chemformula_fontspec_aux:w [#1]#2 \q_stop
  { \newfontfamily \chemformula_font: [#1] {#2} }

\cs_new_protected:Npn \__chemformula_save_catcodes:
  {
    \cs_gset_protected:Npx \__chemformula_restore:
      {
        \char_set_catcode:nn {  95 } { \char_value_catcode:n {  95 } }% _
        \char_set_catcode:nn {  58 } { \char_value_catcode:n {  58 } }% :
        \char_set_catcode:nn {  94 } { \char_value_catcode:n {  94 } }% ^
        \char_set_catcode:nn { 123 } { \char_value_catcode:n { 123 } }% {
        \char_set_catcode:nn { 125 } { \char_value_catcode:n { 125 } }% }
        \char_set_catcode:nn {  35 } { \char_value_catcode:n {  35 } }% #
        \char_set_catcode:nn {  33 } { \char_value_catcode:n {  33 } }% !
        \char_set_catcode:nn {  34 } { \char_value_catcode:n {  34 } }% "
        \char_set_catcode:nn {  60 } { \char_value_catcode:n {  60 } }% <
        \char_set_catcode:nn {  62 } { \char_value_catcode:n {  62 } }% >
        \char_set_catcode:nn { 126 } { \char_value_catcode:n { 126 } }% ~
      }
  }

\cs_new_protected:Npn \__chemformula_sanitize:Nn #1#2
  {
    \tl_set_rescan:Nnn #1
      {
        % expl3 catcodes:
        \char_set_catcode_letter:N \_
        \char_set_catcode_letter:N \:
        % I've forgotten why I put this here:
        \char_set_catcode_math_superscript:N \^
        % disable some of the specials (needed for sub- and superscripts):
        \char_set_catcode_letter:N \{
        \char_set_catcode_letter:N \}
        % this one seems not necessary any more:
        % \char_set_catcode_letter:N \\
        \char_set_catcode_letter:N \#
        % make the naming work with babel languages like French that make
        % ! active:
        \char_set_catcode_other:N  \!
        % make the naming work with babel languages like Spanish that make
        % < and > active:
        \char_set_catcode_other:N \>
        \char_set_catcode_other:N \<
        % ensure " is other:
        \char_set_catcode_other:N \"
      }
      {#2}
  }
\cs_generate_variant:Nn \__chemformula_sanitize:Nn {NV}

\cs_new_protected:Npn \__chemformula_prepare_output:Nn #1#2
  {
    \tl_set_rescan:NnV #1
      {
        \__chemformula_restore:
        #2
        % we /need/ those:
        \char_set_catcode_letter:N \@
        \char_set_catcode_letter:N \_
        \char_set_catcode_letter:N \:
      }
      #1
  }
\cs_generate_variant:Nn \__chemformula_prepare_output:Nn {NV}
  
\cs_new_protected:Npn \__chemformula_reset:Nn #1#2
  { \tl_set_rescan:Nnn #1 { \__chemformula_restore: } {#2} }
\cs_generate_variant:Nn \__chemformula_reset:Nn {NV}

\prg_new_protected_conditional:Npnn \__chemformula_if_only_parens:n #1 {T,F,TF}
  {
    \bool_set_true:N \l__chemformula_tmpa_bool
    \tl_map_inline:nn {#1}
      {
        \str_case:nnF {##1}
          {
            {[} { }
            {]} { }
            {(} { }
            {)} { }
          }
          { \bool_set_false:N \l__chemformula_tmpa_bool }
      }
    \bool_if:NTF \l__chemformula_tmpa_bool
      { \prg_return_true: }
      { \prg_return_false: }
  }

% if the compound starts with a number or a sub- or superscript then it is an
% isotope:
\cs_new_protected:Npn \__chemformula_check_for_isotope:N #1
  {
    \tl_if_eq:xnT { \tl_head:V #1 } {^}
      {
        \tl_put_left:Nn #1
          { \bool_set_true:N \l__chemformula_is_isotope_bool }
      }
    \tl_if_eq:xnT { \tl_head:V #1 } {_}
      {
        \tl_put_left:Nn #1
          { \bool_set_true:N \l__chemformula_is_isotope_bool }
      }
    \prop_if_in:NxT \l__chemformula_numbers_prop
      { \tl_head:V #1 }
      {
        \tl_put_left:Nn #1
          { \bool_set_true:N \l__chemformula_is_isotope_bool }
      }
  }

\cs_new_protected:Npn \__chemformula_prepare_sub_and_superscripts:N #1
  {
    \tl_if_in:VnT #1 {^}
      { \tl_replace_all:Nnn #1 {^} { \chemformula_superscript:n } }
    \tl_if_in:VnT #1 {_}
      { \tl_replace_all:Nnn #1 {_} { \chemformula_subscript:n } }
  }

% #1: tl that holds compound
% #2: code for each token
\cs_new_protected:Npn \__chemformula_compound_map_inline:Nn #1#2
  {
    % ##1: token
    % ##2: plus case
    % ##3: minus case
    % ##4: else case
    \cs_set_protected:Npn \__chemformula_if_last_token_charge:nnnF ##1##2##3##4
      {
        \int_compare:nTF
          { \l__chemformula_count_tokens_int = \tl_count:N #1 }
          {
            \tl_if_eq:nnTF {##1} {+}
              {##2}
              {
                \tl_if_eq:nnTF {##1} {-}
                  {##3}
                  {##4}
              }
          }
          {##4}
      }
    \int_zero:N \l__chemformula_count_tokens_int
    \tl_map_inline:Nn #1
      {
        \int_incr:N \l__chemformula_count_tokens_int
        % do we end with a charge?
        #2
      }
  }

% this is the chemical formula:
% #1: tl var to safe processed formula in
% #2: formula
\cs_new_protected:Npn \__chemformula_input_compound_no_check:Nn #1#2
  {
    \__chemformula_if_only_parens:nTF {#2}
      { \tl_put_right:Nn #1 {#2} }
      {
        % start with formatting:
        \tl_put_right:Nn #1
          {
            \tl_use:N \l__chemformula_format_tl
            \group_begin:
              \l__chemformula_atom_format_tl
          }
        \__chemformula_sanitize:Nn \l__chemformula_tmpa_tl {#2}
        % let's see if the compound starts with a sub- or superscript - if yes
        % this is an isotope:
        \__chemformula_check_for_isotope:N \l__chemformula_tmpa_tl
        % preparations for sub- and superscripting:
        \__chemformula_prepare_sub_and_superscripts:N \l__chemformula_tmpa_tl
        \__chemformula_compound_map_inline:Nn \l__chemformula_tmpa_tl
          {
            % do we end with a charge?
            \__chemformula_if_last_token_charge:nnnF {##1}
              {
                \tl_put_right:Nn #1
                  { \chemformula_superscript:n { \chemformula_plus: } }
              }
              {
                \tl_put_right:Nn #1
                  { \chemformula_superscript:n { \chemformula_minus: } }
              }
              {
                \prop_get:NnNTF \l__chemformula_compound_prop
                  {##1}
                  \l__chemformula_tmpb_tl
                  { \tl_put_right:NV #1 \l__chemformula_tmpb_tl }
                  { \tl_put_right:Nn #1 {##1} }
              }
          }
        % if Kroeger-Vink notation is active:
        \bool_if:NT \l__chemformula_kroegervink_bool
          {
            \tl_replace_all:Nnn #1
              { \chemformula_prime: }
              { \chemformula_superscript:n { \chemformula_prime: } }
          }
        % finish formatting:
        \tl_put_right:Nn #1 { \group_end: }
      }
     \tl_clear:N \l__chemformula_tmpa_tl
  }
\cs_generate_variant:Nn \__chemformula_input_compound_no_check:Nn {NV}

% --------------------------------------------------------------------------
% clean up chemmacros commands:
\cs_new_protected:Npn \__chemformula_clean_chemmacros:n #1
  {
    \cs_set:Nn \chemmacros_text:n
      { \mode_if_math:TF { \text {##1} } {##1} }
    \cs_set:Npn \chemmacros_xspace: {}
  }
\cs_generate_variant:Nn \__chemformula_clean_chemmacros:n {V}

% --------------------------------------------------------------------------
% input "and" sign
\prop_new:N \l__chemformula_addition_symbol_prop
\skip_new:N \l__chemformula_additions_symbol_space_skip
\tl_new:N \l__chemformula_additions_symbol_penalty_tl

\skip_set:Nn \l__chemformula_additions_symbol_space_skip
  {.3em plus .1em minus .1em}
\tl_set:Nn \l__chemformula_additions_symbol_penalty_tl {700}

% #1: name
% #2: input symbol
% #3: output symbol
\cs_new_protected:Npn \chemformula_new_addition_symbol:nnn #1#2#3
  {
    \prop_put:Nnn \l__chemformula_addition_symbol_prop {#1} {#2}
    \skip_zero_new:c {l__chemformula_#1_space_skip}
    \skip_set:cV     {l__chemformula_#1_space_skip}
      \l__chemformula_additions_symbol_space_skip
    \tl_clear_new:c  {l__chemformula_#1_penalty_tl}
    \tl_set:cV       {l__chemformula_#1_penalty_tl}
      \l__chemformula_additions_symbol_penalty_tl
    \tl_clear_new:c  {l__chemformula_#1_output_tl}
    \tl_set:cn       {l__chemformula_#1_output_tl} {#3}
    \keys_define:nn {chemformula}
      {
        #1-space         .skip_set:c = {l__chemformula_#1_space_skip} ,
        #1-penalty       .tl_set:c   = {l__chemformula_#1_penalty_tl} ,
        #1-output-symbol .tl_set:c   = {l__chemformula_#1_output_tl}
      }
  }

% #1: name
\cs_new_protected:Npn \chemformula_get_addition_symbol:n #1
  {
    \chemformula_skip_nobreak:c {l__chemformula_#1_space_skip}
    \tl_use:c {l__chemformula_#1_output_tl}
    \chemformula_skip_nobreak_penalty:cn
      {l__chemformula_#1_space_skip}
      { \use:c {l__chemformula_#1_penalty_tl} }
  }

% #1: name
\cs_new_protected:Npn \__chemformula_input_addition:n #1
  {
    \prop_map_inline:Nn \l__chemformula_addition_symbol_prop
      {
        \str_if_eq:nnT {#1} {##2}
          {
            \tl_put_right:Nn \l__chemformula_input_tl
              { \chemformula_get_addition_symbol:n {##1} }
            \prop_map_break:
          }
      }
  }

% #1: input string
\cs_new_protected:Npn \__chemformula_detect_addition_symbols:n #1
  {
    \bool_set_false:N \l__chemformula_is_addition_symbol_bool
    \chemformula_is_addition_symbol_input:nT {#1}
      { \bool_set_true:N \l__chemformula_is_addition_symbol_bool }
  }

\prg_new_protected_conditional:Npnn \chemformula_is_addition_symbol_input:n #1 {T,F,TF}
  {
    \bool_set_false:N \l__chemformula_tmpa_bool
    \prop_map_inline:Nn \l__chemformula_addition_symbol_prop
      {
        \str_if_eq:nnT {#1} {##2}
          {
            \bool_set_true:N \l__chemformula_tmpa_bool
            \prop_map_break:
          }
      }
    \bool_if:NTF \l__chemformula_tmpa_bool
      { \prg_return_true: }
      { \prg_return_false: }
  }

\prg_new_conditional:Npnn \chemformula_is_addition_symbol_name:n #1 {T,F,TF}
  {
    \prop_if_in:NnTF \l__chemformula_addition_symbol_prop {#1}
      { \prg_return_true: }
      { \prg_return_false: }
  }

\NewDocumentCommand \NewChemAdditionSymbol {mmm}
  {
    \chemformula_is_addition_symbol_name:nT {#1}
      { \msg_error:nnnn {chemformula} {new-addition-symbol} {#1} {#2} }
    \chemformula_is_addition_symbol_input:nT {#2}
      { \msg_error:nnnn {chemformula} {new-addition-symbol} {#1} {#2} }
    \chemformula_new_addition_symbol:nnn {#1} {#2} {#3}
  }

\NewDocumentCommand \ProvideChemAdditionSymbol {mmm}
  {
    \chemformula_is_addition_symbol_name:nF {#1}
      {
        \chemformula_is_addition_symbol_input:nF {#2}
          { \chemformula_new_addition_symbol:nnn {#1} {#2} {#3} }
      }
  }
  
\NewDocumentCommand \RenewChemAdditionSymbol {mmm}
  {
    \chemformula_is_addition_symbol_name:nF {#1}
      { \msg_error:nnnn {chemformula} {renew-addition-symbol} {#1} {#2} }
    \chemformula_is_addition_symbol_input:nF {#2}
      { \msg_error:nnnn {chemformula} {renew-addition-symbol} {#1} {#2} }
    \chemformula_new_addition_symbol:nnn {#1} {#2} {#3}
  }

\NewDocumentCommand \DeclareChemAdditionSymbol {mmm}
  { \chemformula_new_addition_symbol:nnn {#1} {#2} {#3} }

% default addition symbols:
\NewChemAdditionSymbol {plus}  {+} {+}
\NewChemAdditionSymbol {minus} {-} {$-$}

% --------------------------------------------------------------------------
% input escaping gas and precipitate
\prop_new:N \l__chemformula_symbols_prop

\cs_new_protected:Npn \__chemformula_input_symbol:n #1
  {
    \prop_map_inline:Nn \l__chemformula_symbols_prop
      {
        \str_if_eq:nnT {#1} {##1}
          {
            \tl_put_right:Nn \l__chemformula_input_tl {##2}
            \prop_map_break:
          }
      }
    \bool_set_false:N \l__chemformula_is_symbol_bool
  }

\cs_new_protected:Npn \chemformula_new_symbol:nn #1#2
  { \prop_put:Nnn \l__chemformula_symbols_prop {#1} {#2} }
  
\cs_new_protected:Npn \__chemformula_detect_symbols:n #1
  {
    \bool_set_false:N \l__chemformula_is_symbol_bool
    \chemformula_is_symbol:nT {#1}
      { \bool_set_true:N \l__chemformula_is_symbol_bool }
  }

\prg_new_conditional:Npnn \chemformula_is_symbol:n #1 {T,F,TF}
  {
    \prop_if_in:NnTF \l__chemformula_symbols_prop {#1}
      { \prg_return_true: }
      { \prg_return_false: }
  }

\NewDocumentCommand \NewChemSymbol {mm}
  {
    \chemformula_is_symbol:nTF {#1}
      { \msg_error:nnn {chemformula} {new-symbol} {#1} }
      { \chemformula_new_symbol:nn {#1} {#2} }
  }

\NewDocumentCommand \ProvideChemSymbol {mm}
  {
    \chemformula_is_symbol:nF {#1}
      { \chemformula_new_symbol:nn {#1} {#2} }
  }
  
\NewDocumentCommand \RenewChemSymbol {mm}
  {
    \chemformula_is_symbol:nTF {#1}
      { \chemformula_new_symbol:nn {#1} {#2} }
      { \msg_error:nnn {chemformula} {renew-symbol} {#1} }
  }

\NewDocumentCommand \DeclareChemSymbol {mm}
  { \chemformula_new_symbol:nn {#1} {#2} }
  
\NewChemSymbol {v} {$\downarrow$}
\NewChemSymbol {^} {$\uparrow$}

% --------------------------------------------------------------------------
% input arrow
\cs_new_protected:Npn \__chemformula_input_arrow:n #1
  {
    \bool_if:nT
      {
        \l__chemformula_is_arrow_bool &&
        !\l__chemformula_first_last_text_bool &&
        !\l__chemformula_first_last_math_bool
      }
      {
        \__chemformula_generate_arrows:Nn \l__chemformula_tmpa_tl {#1}
        \tl_put_right:NV \l__chemformula_input_tl \l__chemformula_tmpa_tl
      }
    \bool_set_false:N \l__chemformula_is_arrow_bool
  }
\cs_generate_variant:Nn \__chemformula_input_arrow:n {o,V}

% --------------------------------------------------------------------------
% input compound name
\cs_new_protected:Npn \__chemformula_input_name:n #1
  {
    \bool_if:nT
      {
        \l__chemformula_is_name_bool &&
        !\l__chemformula_first_last_text_bool &&
        !\l__chemformula_first_last_math_bool
      }
      {
        \__chemformula_generate_name:Nn \l__chemformula_tmpa_tl {#1}
        \tl_put_right:NV \l__chemformula_input_tl \l__chemformula_tmpa_tl
      }
    \bool_set_false:N \l__chemformula_is_name_bool
  }

% --------------------------------------------------------------------------
% input escaped text
% #1: <bool> #2: <tl> #3: <first> #4: <last>
\cs_new_protected:Npn \__chemformula_bool_set_if_first_last:Nnnn #1#2#3#4
  {
    \int_zero:N \l__chemformula_tmpa_int
    \int_zero:N \l__chemformula_tmpb_int
    \int_set:Nn \l__chemformula_tmpa_int { \tl_count:n {#2} }
    \tl_map_inline:nn {#2}
      {
        \int_incr:N \l__chemformula_tmpb_int
        \int_compare:nT { \l__chemformula_tmpb_int = 1 }
          {
            \tl_if_eq:nnT {##1} {#3}
              { \bool_set_true:N #1 }
          }
        \int_compare:nT { \l__chemformula_tmpb_int = \l__chemformula_tmpa_int }
          {
            \tl_if_eq:nnF {##1} {#4}
              { \bool_set_false:N #1 }
          }
      }
  }

\cs_new_protected:Npn \__chemformula_input_escape_text:n #1
  {
    \__chemformula_first_last_text:n {#1}
    \bool_if:NT \l__chemformula_first_last_double_bool
      {
        \bool_set_true:N \l__chemformula_first_last_text_bool
        \__chemformula_read_escape_double:w #1 \q_nil
      }
    \bool_if:NT \l__chemformula_first_last_single_bool
      {
        \bool_set_true:N \l__chemformula_first_last_text_bool
        \__chemformula_read_escape_single:w #1 \q_nil
      }
  }
\cs_generate_variant:Nn \__chemformula_input_escape_text:n {V}

\cs_new_protected:Npn \__chemformula_read_escape_text:n #1
  {
    \tl_set_rescan:Nnn \l__chemformula_tmpa_tl
      {
        \char_set_catcode_letter:N \_
        \char_set_catcode_other:N \:
      }
      {#1}
    \tl_replace_all:Nnn \l__chemformula_tmpa_tl
      { \_ } { chemformulaplaceholder }
    \tl_replace_all:Nnn \l__chemformula_tmpa_tl
      {_} { \sb }
    \tl_replace_all:Nnn \l__chemformula_tmpa_tl
      { chemformulaplaceholder } { \_ }
    % those cause trouble with chemmacros' \ox:
    % \tl_put_left:Nn \l__chemformula_tmpa_tl
    %   { \chemformula_group_begin: }
    % \tl_put_right:Nn \l__chemformula_tmpa_tl
    % { \chemformula_group_end: }
    % \tl_set_rescan:NnV \l__chemformula_tmpa_tl
    %   { \__chemformula_restore: \char_set_catcode_other:N \~ }
    %   \l__chemformula_tmpa_tl
    % \__chemformula_reset:NV \l__chemformula_tmpa_tl \l__chemformula_tmpa_tl
    \tl_put_right:NV \l__chemformula_input_tl \l__chemformula_tmpa_tl
  }

\cs_new_protected:Npn \__chemformula_read_escape_double:w "#1" \q_nil
  { \__chemformula_read_escape_text:n {#1} }

\cs_new_protected:Npn \__chemformula_read_escape_single:w '#1' \q_nil
  { \__chemformula_read_escape_text:n {#1} }

\cs_new_protected:Npn \__chemformula_first_last_text:n #1
  {
    \bool_set_false:N \l__chemformula_first_last_double_bool
    \bool_set_false:N \l__chemformula_first_last_single_bool
    \bool_set_false:N \l__chemformula_first_last_text_bool
    \__chemformula_bool_set_if_first_last:Nnnn
      \l__chemformula_first_last_double_bool
      {#1}
      { " } { " }
    \bool_if:NF \l__chemformula_first_last_double_bool
      {
        \__chemformula_bool_set_if_first_last:Nnnn
          \l__chemformula_first_last_single_bool
          {#1}
          { ' } { ' }
      }
  }

% --------------------------------------------------------------------------
% input escaped math
\cs_new_protected:Npn \__chemformula_input_escape_math:n #1
  {
    \__chemformula_first_last_math:n {#1}
    \bool_if:NT \l__chemformula_first_last_dollar_bool
      {
        \bool_set_true:N \l__chemformula_first_last_math_bool
        \__chemformula_read_escape_dollar:w #1 \q_nil
      }
    \bool_if:NT \l__chemformula_first_last_mathbraces_bool
      {
        \bool_set_true:N \l__chemformula_first_last_math_bool
        \__chemformula_read_escape_mathbraces:w #1 \q_nil
      }
  }

\cs_new_protected:Npn \__chemformula_read_escape_math:n #1
  {
    \tl_set_rescan:Nnn \l__chemformula_tmpa_tl
      {
        \char_set_catcode_letter:N \_
        \char_set_catcode_letter:N \:
      }
      { \c_math_toggle_token #1 \c_math_toggle_token }
    \tl_replace_all:Nnn \l__chemformula_tmpa_tl
      { \_ } { chemformulaplaceholder }
    \tl_replace_all:Nnn \l__chemformula_tmpa_tl
      {_} { \sb }
    \tl_replace_all:Nnn \l__chemformula_tmpa_tl
      { chemformulaplaceholder } { \_ }
    % \__chemformula_reset:NV \l__chemformula_tmpa_tl \l__chemformula_tmpa_tl
    \tl_put_left:Nn \l__chemformula_tmpa_tl
      { \chemformula_group_begin: }
    \tl_put_right:Nn \l__chemformula_tmpa_tl
      { \chemformula_group_end: }
    \tl_put_right:NV \l__chemformula_input_tl \l__chemformula_tmpa_tl
    \tl_put_right:Nn \l__chemformula_input_tl
      { \chemformula_skip_nobreak:N \l__chemformula_math_space_skip }
  }

\cs_new_protected:Npn \__chemformula_read_escape_dollar:w $#1$ \q_nil
  { \__chemformula_read_escape_math:n {#1} }

\cs_new_protected:Npn \__chemformula_read_escape_mathbraces:w \(#1\) \q_nil
  { \__chemformula_read_escape_math:n {#1} }

\cs_new_protected:Npn \__chemformula_first_last_math:n #1
  {
    \bool_set_false:N \l__chemformula_first_last_math_bool
    \bool_set_false:N \l__chemformula_first_last_dollar_bool
    \bool_set_false:N \l__chemformula_first_last_mathbraces_bool
    \__chemformula_bool_set_if_first_last:Nnnn
      \l__chemformula_first_last_dollar_bool
      {#1}
      { $ } { $ }
    \bool_if:NF \l__chemformula_first_last_dollar_bool
      {
        \__chemformula_bool_set_if_first_last:Nnnn
          \l__chemformula_first_last_mathbraces_bool
          {#1}
          { \( } { \) }
      }
  }

% --------------------------------------------------------------------------
% customization of output other than arrows
\keys_define:nn {chemformula}
  {
    circled            .choice: ,
    circled / none     .code:n =
      { \bool_set_false:N \l_chemformula_formal_charges_bool } ,
    circled / formal   .code:n =
      { \bool_set_false:N \l_chemformula_formal_charges_bool } ,
    circled / all      .code:n =
      { \bool_set_true:N \l_chemformula_formal_charges_bool  } ,
    circled            .default:n  = all ,
    circletype         .choice: ,
    circletype / math  .code:n  =
      { \bool_set_false:N \l_chemformula_formal_chem_bool } ,
    circletype / chem  .code:n  =
      { \bool_set_true:N \l_chemformula_formal_chem_bool } ,
    circletype         .default:n = chem ,
    charge-hshift         .code:n    =
      \tl_if_eq:nnTF {#1} { full }
        { \bool_set_true:N \l__chemformula_charge_full_shift_bool }
        {
          \bool_set_false:N \l__chemformula_charge_full_shift_bool
          \dim_set:Nn \l__chemformula_charge_shift_dim {#1}
        } ,
    charge-hshift         .default:n = .25em ,
    charge-vshift         .tl_set:N =
      \l__chemformula_superscript_shift_additional_tl ,
    charge-style          .choice: ,
    charge-style / text   .code:n    =
      \bool_set_false:N \l__chemformula_charge_style_math_bool ,
    charge-style / math   .code:n    =
      \bool_set_true:N \l__chemformula_charge_style_math_bool ,
    subscript-vshift      .tl_set:N =
      \l__chemformula_subscript_shift_additional_tl ,
    subscript-vshift      .default:n = 0pt ,
    subscript-style       .choice: ,
    subscript-style / text .code:n   =
      \bool_set_false:N \l__chemformula_number_style_math_bool ,
    subscript-style / math .code:n   =
      \bool_set_true:N \l__chemformula_number_style_math_bool ,
    decimal-marker        .tl_set:N  = \l__chemformula_decimal_output_tl ,
    frac-style            .choice: ,
    frac-style / math     .code:n    =
      \bool_set_false:N \l__chemformula_xfrac_bool
      \bool_set_false:N \l__chemformula_nicefrac_bool
      \bool_set_true:N \l__chemformula_mathfrac_bool ,
    frac-style / xfrac    .code:n    =
      \bool_set_true:N \l__chemformula_xfrac_bool
      \bool_set_false:N \l__chemformula_nicefrac_bool
      \bool_set_false:N \l__chemformula_mathfrac_bool ,
    frac-style / nicefrac .code:n    =
      \bool_set_false:N \l__chemformula_xfrac_bool
      \bool_set_true:N \l__chemformula_nicefrac_bool
      \bool_set_false:N \l__chemformula_mathfrac_bool ,
    frac-math-cmd         .code:n     =
      { \cs_set_eq:NN \chemformula_math_frac:nn #1 } ,
    stoich-paren-parse    .bool_set:N = \l__chemformula_stoich_parse_iupac_bool ,
    stoich-space          .skip_set:N = \l__chemformula_stoich_space_skip ,
    stoich-print          .code:n     =
      \cs_set_eq:NN \chemformula_print_stoich:n #1 ,
    adduct-space          .dim_set:N  = \l__chemformula_adduct_space_dim ,
    adduct-penalty        .tl_set:N   = \l__chemformula_adduct_penalty_tl ,
    math-space            .skip_set:N = \l__chemformula_math_space_skip ,
    name-format           .tl_set:N   = \l__chemformula_name_format_tl ,
    name-width            .code:n     =
      {
        \tl_if_eq:nnTF {#1} { auto }
          { \bool_set_false:N \l__chemformula_name_width_bool }
          {
            \bool_set_true:N \l__chemformula_name_width_bool
            \dim_set:Nn \l__chemformula_name_dim {#1}
          }
      } ,
    bond-length           .dim_set:N = \l__chemformula_bond_dim ,
    bond-offset           .dim_set:N = \l__chemformula_bond_space_dim ,
    bond-style            .code:n    =
      \tikzset { chembond/.style={Butt~Cap-Butt~Cap,#1} } ,
    bond-penalty          .tl_set:N  = \l__chemformula_bond_penalty_tl ,
    font-family           .code:n    =
      \bool_set_true:N \l__chemformula_fss_bool
      \bool_set_false:N \l__chemformula_fontspec_bool
      \bool_set_true:N \l__chemformula_fss_family_bool
      \tl_set:Nn \l__chemformula_font_family_tl {#1} ,
    font-series           .code:n    =
      \bool_set_true:N \l__chemformula_fss_bool
      \bool_set_false:N \l__chemformula_fontspec_bool
      \bool_set_true:N \l__chemformula_fss_series_bool
      \tl_set:Nn \l__chemformula_font_series_tl {#1} ,
    font-shape            .code:n    =
      \bool_set_true:N \l__chemformula_fss_bool
      \bool_set_false:N \l__chemformula_fontspec_bool
      \bool_set_true:N \l__chemformula_fss_shape_bool
      \tl_set:Nn \l__chemformula_font_shape_tl {#1} ,
    font-spec             .code:n    =
      \bool_set_false:N \l__chemformula_fss_bool
      \bool_set_true:N \l__chemformula_fontspec_bool
      \__chemformula_fontspec:n {#1} ,
    format                .code:n    =
      \bool_set_false:N \l__chemformula_fss_bool
      \bool_set_false:N \l__chemformula_fontspec_bool
      \tl_set:Nn \l__chemformula_format_tl {#1} ,
    atom-format           .tl_set:N  = \l__chemformula_atom_format_tl ,
    set-catcodes          .tl_set:N  = \l__chemformula_catcodes_tl  
  }

% --------------------------------------------------------------------------
% \chlewis[opions]{el spec}{atom}

\keys_define:nn {chemformula}
  {
    lewis-default     .tl_set:N  = \l__chemformula_elspec_default_type_tl ,
    lewis-distance    .dim_set:N = \l__chemformula_elspec_pair_distance_dim ,
    lewis-line-length .dim_set:N = \l__chemformula_elspec_pair_line_length_dim ,
    lewis-line-width  .dim_set:N = \l__chemformula_elspec_pair_width_dim ,
    lewis-offset      .tl_set:N  = \l__chemformula_elspec_radius_tl
  }

% #1: options
% #2- el spec
% #3: atom
\cs_new_protected:Npn \chemformula_lewis:nnn #1#2#3
  {
    \group_begin:
      \keys_set:nn {chemformula} {#1}
      \int_gincr:N \g__chemformula_lewis_int
      \chemformula_get_lewis_sequence:Nn \l__chemformula_tmpa_seq {#2}
      \chemformula_tikz:nn { baseline }
        {
          \chemformula_node:nxn { anchor=base , inner~sep=0pt }
            { chlewis-\int_use:N \g__chemformula_lewis_int }
            {
              \bool_if:NTF \l_chemformula_inside_ch_bool
                {#3}
                { \chemformula_chcpd:nn {} {#3} }
            }
          \seq_map_inline:Nn \l__chemformula_tmpa_seq
            { \chemformula_lewis_get_angle_and_spec:n {##1} }
        }
    \group_end:
  }

\cs_new_protected:Npn \chemformula_get_lewis_sequence:Nn #1#2
  {
    \tl_set:Nn \l__chemformula_tmpa_tl {#2}
    \tl_replace_all:Nnn \l__chemformula_tmpa_tl {:} { \scan_stop: : , }
    \tl_replace_all:NVn \l__chemformula_tmpa_tl \c__chemformula_colon_tl
      { \scan_stop: : , }
    \tl_replace_all:Nnn \l__chemformula_tmpa_tl {.} { \scan_stop: . , }
    \tl_replace_all:Nnn \l__chemformula_tmpa_tl {|} { \scan_stop: | , }
    \tl_replace_all:Nnn \l__chemformula_tmpa_tl {o} { \scan_stop: o , }
    \seq_set_split:NnV #1 {,} \l__chemformula_tmpa_tl
  }

\cs_new_protected:Npn \chemformula_lewis_get_angle_and_spec:n #1
  { \__chemformula_lewis_get_angle_and_spec:w #1 \scan_stop: \scan_stop: \q_stop }

\cs_new_protected:Npn
  \__chemformula_lewis_get_angle_and_spec:w #1 \scan_stop: #2 \scan_stop: #3 \q_stop
  {
    \tl_if_blank:nF {#1}
      {
        \tl_set:Nn \l__chemformula_elspec_angle_tl {#1}
        \tl_if_blank:nTF {#2}
          {
            \tl_set_eq:NN
              \l__chemformula_elspec_type_tl
              \l__chemformula_elspec_default_type_tl
          }
          { \tl_set:Nn \l__chemformula_elspec_type_tl {#2} }
        \__chemformula_lewis_set_elspec_type:V \l__chemformula_elspec_type_tl
      }
  }

\cs_new_protected:Npn \__chemformula_lewis_set_elspec_type:n #1
  {
    \prop_get:NnNT \l__chemformula_lewis_elspec_types_prop {#1}
      \l__chemformula_tmpa_tl
      { \tl_use:N \l__chemformula_tmpa_tl }
  }
\cs_generate_variant:Nn \__chemformula_lewis_set_elspec_type:n {V}

% available types:
\cs_new_protected:Npn \chemformula_declare_lewis_elspec_type:nn #1#2
  { \prop_put:Nnn \l__chemformula_lewis_elspec_types_prop {#1} {#2} }

\chemformula_declare_lewis_elspec_type:nn
  {|}
  { \__chemformula_lewis_elspec_pair_line: }

\chemformula_declare_lewis_elspec_type:nn
  {:}
  { \__chemformula_lewis_elspec_pair_dots: }

\chemformula_declare_lewis_elspec_type:nn
  {.}
  { \__chemformula_lewis_elspec_single: }

\chemformula_declare_lewis_elspec_type:nn
  {o}
  { \__chemformula_lewis_elspec_empty: }

\chemformula_declare_lewis_elspec_type:nn
  {pair}
  { \__chemformula_lewis_elspec_pair_dots: }

\chemformula_declare_lewis_elspec_type:nn
  {pair~(dotted)}
  { \__chemformula_lewis_elspec_pair_dots: }

\chemformula_declare_lewis_elspec_type:nn
  {pair~(line)}
  { \__chemformula_lewis_elspec_pair_line: }

\chemformula_declare_lewis_elspec_type:nn
  {single}
  { \__chemformula_lewis_elspec_single: }

\chemformula_declare_lewis_elspec_type:nn
  {empty}
  { \__chemformula_lewis_elspec_empty: }

% functions that do the actual drawing:
\cs_new_protected:Npn \__chemformula_lewis_elspec_pair_dots:
  {
    \chemformula_draw:nn
      {
        fill,
        inner~sep=0pt,
        outer~sep=0pt
      }
      {
        (node~cs \c__chemformula_colon_tl
          name  = chlewis-\int_use:N \g__chemformula_lewis_int ,
          angle = \l__chemformula_elspec_angle_tl
        )
        ++
        (
          \l__chemformula_elspec_angle_tl
          \c__chemformula_colon_tl
          \l__chemformula_elspec_radius_tl
        )
        ++
        (
          \l__chemformula_elspec_angle_tl +90
          \c__chemformula_colon_tl
          .5\l__chemformula_elspec_pair_distance_dim
        )
        circle (\l__chemformula_radical_radius_dim)
        ++
        (
          \l__chemformula_elspec_angle_tl -90
          \c__chemformula_colon_tl
          \l__chemformula_elspec_pair_distance_dim
        )
        circle (\l__chemformula_radical_radius_dim)
      }
  }

\cs_new_protected:Npn \__chemformula_lewis_elspec_pair_line:
  {
    \chemformula_draw:nn
      { line~width = \l__chemformula_elspec_pair_width_dim }
      {
        (node~cs \c__chemformula_colon_tl
          name  = chlewis-\int_use:N \g__chemformula_lewis_int ,
          angle = \l__chemformula_elspec_angle_tl
        )
        ++
        (
          \l__chemformula_elspec_angle_tl
          \c__chemformula_colon_tl
          \l__chemformula_elspec_radius_tl
        )
        ++
        (
          \l__chemformula_elspec_angle_tl +90
          \c__chemformula_colon_tl
          .5\l__chemformula_elspec_pair_line_length_dim
        )
        --
        ++
        (
          \l__chemformula_elspec_angle_tl -90
          \c__chemformula_colon_tl
          \l__chemformula_elspec_pair_line_length_dim
        )
      }
  }

\cs_new_protected:Npn \__chemformula_lewis_elspec_single:
  {
    \chemformula_draw:nn
      {
        fill,
        inner~sep=0pt,
        outer~sep=0pt
      }
      {
        (node~cs \c__chemformula_colon_tl
          name  = chlewis-\int_use:N \g__chemformula_lewis_int ,
          angle = \l__chemformula_elspec_angle_tl
        )
        ++
        (
          \l__chemformula_elspec_angle_tl
          \c__chemformula_colon_tl
          \l__chemformula_elspec_radius_tl
        )
        circle (\l__chemformula_radical_radius_dim)
      }
  }

\cs_new_protected:Npn \__chemformula_lewis_elspec_empty:
  {
    \chemformula_draw:nn { }
      {
        (node~cs \c__chemformula_colon_tl
          name  = chlewis-\int_use:N \g__chemformula_lewis_int ,
          angle = \l__chemformula_elspec_angle_tl
        )
        ++
        (
          \l__chemformula_elspec_angle_tl
          \c__chemformula_colon_tl
          \l__chemformula_elspec_radius_tl
        )
        ++
        (
          \l__chemformula_elspec_angle_tl +90
          \c__chemformula_colon_tl
          .5\l__chemformula_elspec_pair_line_length_dim
        )
        coordinate (chlewis-tmp1)
        ++
        (
          \l__chemformula_elspec_angle_tl -90
          \c__chemformula_colon_tl
          \l__chemformula_elspec_pair_line_length_dim
        )
        coordinate (chlewis-tmp2)
        ++
        (
          \l__chemformula_elspec_angle_tl
          \c__chemformula_colon_tl
          .25\l__chemformula_elspec_pair_line_length_dim
        )
        coordinate (chlewis-tmp3)
        ++
        (
          \l__chemformula_elspec_angle_tl +90
          \c__chemformula_colon_tl
          \l__chemformula_elspec_pair_line_length_dim
        )
        coordinate (chlewis-tmp4)
        (chlewis-tmp1) -- (chlewis-tmp2) --
        (chlewis-tmp3) -- (chlewis-tmp4) --
        cycle
      }
  }

\NewDocumentCommand\chlewis{O{}mm}
  { \chemformula_lewis:nnn {#1} {#2} {#3} }

% --------------------------------------------------------------------------

\NewDocumentCommand \setchemformula {m}
  { \keys_set:nn {chemformula} {#1} }

\setchemformula{
  bond-style  = {} ,
  arrow-style = {}
}

% --------------------------------------------------------------------------
% hyperref support
\AfterPackage* { hyperref }
  {
    \pdfstringdefDisableCommands
      {
        \cs_set:Npn \chcpd #1 {#1}
        \cs_set:Npn \charrow #1 {#1}
        \cs_set:Npn \chname (#1)(#2) { #2~(#1) }
        \cs_set:Npn \ch #1 {#1}
        \cs_set:Npn \chlewis #1#2 {#2}
      }
  }

\tex_endinput:D

% --------------------------------------------------------------------------
VERSION HISTORY
2012/01/28 - version 3.0   - bundled with packages `chemmacros' and `ghs'
2012/01/30 - version 3.0a  - bugfix
                           - renamed formula => chemformula
                             (following a request by Karl Berry for TeXlive)
2012/02/03 - version 3.0b  - stepped number with `chemmacros'
2012/02/05 - version 3.0c  - stepped number with `chemmacros'
2012/02/10 - version 3.0d  - several bugfixes
                           - new arrow type <>
2012/02/19 - version 3.1   - new arrow types -/>, </- and <o>
                           - new commands \DeclareChemArrow, \RenewChemArrow and
                             \ShowChemArrow
2012/02/26 - version 3.1a  - stepped number with `chemmacros'
2012/03/03 - version 3.1b  - arrow types are stored in a sequence to automate the
                             search and replace in the right order 
                           - new arrow type ==                                   
                           - IUPAC compliance for stoichiometric factors
                           - new handling of super- and subscripts to correct
                             various flaws: clear distinction between isotope and
                             formula
                           - new option charge-vshift
2012/03/14 - version 3.1c  - improved arrow labels
2012/03/20 - version 3.2   - new: option input via @{<options>}
2012/05/07 - version 3.3   - \ch now bypasses \ref{}, \label{}, and \intertext{}
2012/05/13 - version 3.3a  - stepped number with `chemmacros'
2012/05/18 - version 3.3b  - stepped number with `chemmacros'
2012/05/18 - version 3.3c  - stepped number with `chemmacros'
2012/07/24 - version 3.3d  - stepped number with `chemmacros'
                           - default bond length set to 1.1667ex
                           - bug fix in the bonds
2012/08/21 - version 3.3e  - stepped number with `chemmacros'
2012/09/11 - version 3.4   - cleaned a little, minor bug fixes
                           - switched to internal scratch variables
                           - new command \bond, more bond types
                           - new commands \DeclareChemBond, \DeclareChemBondAlias,
                             \RenewChemBond and \ShowChemBond
                           - radical point thicker, new options `radical-style'
                             and `radical-radius'
2012/10/03 - version 3.4a  - stepped number with `chemmacros'
2013/01/04 - version 3.4b  - bug fix in the sub- and superscript handling
2013/01/28 - version 3.5   - disallow breaks at bonds
                           - allow breaks after arrows
                           - made some spaces in chemformulas stretch- and
                             shrinkable
                           - various internal code improvements
                           - changed defaults for horizontal spaces that have in
                             set in ex into corresponding values in em.
                           - improvements to the placement of sub- and superscripts
2013/02/19 - version 3.5a  - stepped number with `chemmacros'
2013/02/26 - version 3.6   - support for use with the `breqn' package
                           - math escaping also via \(\)
                           - internal: clearer escaping macros
                           - new options: `radical-vshift', `radical-hshift'
                             and `radical-space'
                           - bug fix: `radical-...' options are now correctly set
                           - stoichiometric factors: add leading 0 if missing
                           - new option: `stoich-paren-parse'
                           - internal changes: be way more rigid about protected
                             and expandable macros
                           - internal changes: clearer distinction between
                             private macros and accessable module macros
                           - internal changes: clearer distinction between
                             document commands and module level commands
                           - !()() syntax now also works with babel's French
2013/02/27 - version 3.6a  - bug fix: escaped text and math stays escaped, i.e.
                             also arrow code like `<=>'
2013/04/19 - version 3.6b  - new option `arrow-min-length'
                           - bug fix: stoichiometric fractions correctly displayed
2013/07/06 - version 4.0   - don't need `chemmacros' package any more,
                             `chemformula' can be used stand-alone;
                             this has lead to a few definitions that previously
                             were done by chemmacros; chemmacros will get them
                             now from chemformula when needed
                           - bug fix: allow braces in subscripts and superscripts
                             by disabling superfluous \chemformula_subscript:n
                             and \chemformula_superscript:n in the scripts write
                             commands
                           - remove superfluous rescanning at the begin of the
                             formula input; this is handled by the sanitize
                             command later anyway
                           - changed internal mathshifts from $ ... $ to \( ... \)
                           - removed superfluous \textstyle from math-frac
                             definition
                           - new command \charrow{}[][] that allows direct
                             access to chemformula's arrows
                           - new command \chname()() that allows direct access
                             to chemformula's naming
                           - new command \chcpd[]{} that allows typesetting a
                             simple single formula - also not nestable with \ch
                           - new command \setchemformula
                           - package option: `strict'
                           - don't allow a line break before a plus
                             (`chem-and')
                           - don't allow a line break between a stoichiometric
                             factor and a compound
                           - changed formal charges to match ``plain'' charges
2013/07/26 - version 4.0a  - added options `arrow-penalty' (0), `plus-penalty'
                             (700) and `bond-penalty' (10000) that determine if
                             there may be a break after an arrow, a bond or the
                             plus
2013/08/24 - version 4.1   - changed `0' values in TikZ coordinates into `0pt'
                             see http://tex.stackexchange.com/q/128792/5049 and
                             http://tex.stackexchange.com/q/118467/5049 for
                             reference
                           - new option `tikz-external-disable'
                           - new option `frac-math-cmd'
2013/10/24 - version 4.2   - \chemformula_font_inner: is only applied /once/ to
                             a formula, and at the beginning, fixes
                             https://bitbucket.org/cgnieder/chemmacros/issue/23
                           - fixed wrong setting of the TikZ bond style
                           - new option `arrow-style'
                           - new command \chlewis
2013/11/04 - version 4.2a  - bug fix in \chcpd,
                           - change sanitizing/restoring of category codes
2014/01/24 - version 4.3   - changed behaviour of \DeclareChem to match
                             \DeclareRobustCommand, i.e., no check if command
                             is defined; added a \NewChem variant to each that
                             /does/ check; use \NewChem instead of \DeclareChem
                             for definitions
                           - new option stoich-print
                           - new command \chstoich
2014/01/29 - version 4.4   - treat a single `-' as a minus sign
2014/04/07 - version 4.5   - make sure that < and > have catcode 12 when read
                             as parts of arrows
                           - new arrow types `>=<', `>=<<', `>>=<' and `<==>'
                           - arrow lengths and compound offset now correctly
                             adapts to new fontsize if relative lengths (em,ex)
                             are used
                           - allow optional arguments to \\ in equation
                             environments
                           - allow optional arguments to \label in equation
                             environments
                             (credits: http://tex.stackexchange.com/a/166661/5049)
                           - support for Kröger-Vink notation
2014/06/30 - version 4.6   - fix: a comma in a superscript is typeset according
                             to `decimal-marker' again
                           - fix charge type behaviour: option `circled' and
                             `circletype' are now obeyed if the option is
                             changed mid-document
                           - new option `chemformula/circled' similar to
                             `chemmacros/circled'
                           - new option `chemformula/circletype' similar to
                             `chemmacros/circletype'
2014/08/04 - version 4.7   - change dependency: pgf library `arrows.meta'
                             instead of `arrows'
                           - new arrow tip definition allows redeclaration to
                             other existing tips by \pgfkeys{cf/.tip=<tip>}
2014/08/08 - version 4.8   - fix bug introduced in last update: `butt cap' now
                             needs to be `Butt Cap' with the new `arrows.meta'
                             library
                           - split from `chemmacros' bundle as independent
                             package
2014/10/07 - version 4.9   - new options: `minus-space', `minus-penalty',
                             `plus-output-symbol' and `minus-output-symbol'
2015/03/16 - version 4.10  - new macros \NewChemCompoundProperty,
                             \RenewChemCompoundProperty,
                             \DeclareChemCompoundProperty,
                             \RemoveChemCompoundProperty
2015/04/03 - version 4.10a - bug fix in \__chemformula_input_stoich:n
2015/06/30 - version 4.11  - small changes in \chcpd
                           - remove unnecessary \mathchoice => noticable speed
                             improvement
                           - \NewChemAdditionSymbol
                           - \NewChemSymbol
2015/08/12 - version 4.12  - remove chemmacros' `circletype' and `circled'
                             options. They really belong to chemmacros
                           - change requirement amsmath -> amstext
                           - shorten definition of double and triple bonds
2015/08/29 - version 4.12a - \chemformula_formal_plus: ,
                             \chemformula_formal_minus:
                             \ProvideChem... equivalents to all \NewChem...
                             macros 
2015/09/08 - version 4.12b - trim spaces of \ch input -- due to a change in
                             l3kernel
2015/01/01 - version 4.13  - clean up code
                           - check for blank input
                           - drop support for \[ and \]
                           - new option `atom-format'
2015/10/21 - version 4.14  - fix issue
                             https://github.com/cgnieder/chemformula/issues/2
                           - new option `adduct-penalty'
2016/01/07 - version 4.14a - \prop_get:Nn => \prop_item:Nn
2016/05/03 - version 4.14b - local options now are correctly working again
                           - `atom-format' is only applied to formulas
2016/05/11 - version 4.15  - arrow definitions don't have to be made in a
                             certain order any more
                           - new quasi equilibria arrows
                           - bug fix in \RenewChemArrow
                           - meaningful error if \charrow if used with an
                             unknown arrow type
2016/06/08 - version 4.15a - change requirement amstext -> amsmath; \chname
                             uses \underset which needs amsmath
2016/06/08 - version 4.15b - add missing variant of kernel function
2016/08/21 - version 4.15c - corrected redundant definition of
                             \chemformula_single_bond:
                           - fix problem with TikZ externalization and
                             Kroeger-Vink notation
2017/02/02 - version 4.15d - allow `name-format' to end with a macro that
                             takes an argument
2017/03/23 - version 4.15e - small bug fix (issue #5)
                           - implement \shortintertext bypass
2019/09/23 - version 4.15f - add missing definition for temporary variable
2019/09/27 - version 4.15g - bug fix in fontspec option
2019/10/13 - version 4.15h - new (undocumented) option set-catcodes
                           - fix behaviour with category codes
                             (https://tex.stackexchange.com/q/511592/)
                           - fix bug: local format changes are now applied
                             again
2020/02/01 - version 4.15i - fix issue #12

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% IDEAS:
- Addukte: Zahlen nach * und . automatisch erkennen?