siunitx (3may18)

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-% \iffalse meta-comment
-%
-% File: siunitx-unit.dtx Copyright (C) 2014-2018 Joseph Wright
-%
-% It may be distributed and/or modified under the conditions of the
-% LaTeX Project Public License (LPPL), either version 1.3c of this
-% license or (at your option) any later version.  The latest version
-% of this license is in the file
-%
-%    https://www.latex-project.org/lppl.txt
-%
-% This file is part of the "siunitx bundle" (The Work in LPPL)
-% and all files in that bundle must be distributed together.
-%
-% The released version of this bundle is available from CTAN.
-%
-% -----------------------------------------------------------------------
-%
-% The development version of the bundle can be found at
-%
-%    https://github.com/josephwright/siunitx
-%
-% for those people who are interested.
-%
-% -----------------------------------------------------------------------
-%
-%<*driver>
-\documentclass{l3doc}
-% The next line is needed so that \GetFileInfo will be able to pick up
-% version data
-\usepackage{siunitx}
-\begin{document}
-  \DocInput{\jobname.dtx}
-\end{document}
-%</driver>
-% \fi
-%
-% \GetFileInfo{siunitx.sty}
-%
-% \title{^^A
-%   \pkg{siunitx-unit} -- Parsing and formatting units^^A
-%   \thanks{This file describes \fileversion,
-%     last revised \filedate.}^^A
-% }
-%
-% \author{^^A
-%  Joseph Wright^^A
-%  \thanks{^^A
-%    E-mail:
-%    \href{mailto:joseph.wright@morningstar2.co.uk}
-%      {joseph.wright@morningstar2.co.uk}^^A
-%   }^^A
-% }
-%
-% \date{Released \filedate}
-%
-% \maketitle
-%
-% \begin{documentation}
-%
-% This submodule is dedicated to formatting physical units. The main function,
-% \cs{siunitx_unit_format:nN}, takes user input specify physical units and
-% converts it into a formatted token list suitable for typesetting in math
-% mode. While the formatter will deal correctly with \enquote{literal} user
-% input, the key strength of the module is providing a method to describe
-% physical units in a \enquote{symbolic} manner. The output format of these
-% symbolic units can then be controlled by a number of key--value options
-% made available by the module.
-%
-% A small number of \LaTeXe{} math mode commands are assumed to be available
-% as part of the formatted output. The \cs{mathchoice} command
-% (normally the \TeX{} primitive) is needed when using
-% |per-mode = symbol-or-fraction|. The command \cs{mathrm} is used for
-% wrapping the text (letter) part of units. The commands \cs{frac},
-% \cs{mbox}, \verb*|\ | and \cs{,} are used by the standard module settings,
-% and \cs{ensuremath}, \cs{hbar}, \cs{mathit} and \cs{mathrm} in some standard
-% unit definitions (for atomic and natural units). For the display of colored
-% (highlighted) and cancelled units, the commands \cs{textcolor} and
-% \cs{cancel} are assumed to be available.
-%
-% \section{Formatting units}
-%
-% \begin{function}{\siunitx_unit_format:nN}
-%   \begin{syntax}
-%     \cs{siunitx_unit_format:nN} \Arg{units} \meta{tl~var}
-%   \end{syntax}
-%   This function converts the input \meta{units} into a processed
-%   \meta{tl~var} which can then be inserted in math mode to typeset the
-%   material. Where the \meta{units} are given in symbolic form, described
-%   elsewhere, this formatting process takes place in two stages: the
-%   \meta{units} are parsed into a structured form before the generation
-%   of the appropriate output form based on the active settings. When the
-%   \meta{units} are given as literals, processing is minimal: the
-%   characters |.| and |~| are converted to unit products (boundaries).
-%   In both cases, the result is a series of tokens intended to be typeset
-%   in math mode with appropriate choice of font for typesetting of the
-%   textual parts.
-%
-%   For example,
-%   \begin{verbatim}
-%     \siunitx_unit_format:nN { \kilo \metre \per \second } \l_tmpa_tl
-%   \end{verbatim}
-%   will, with standard settings, result in \cs{l_tmpa_tl} being set to
-%   \begin{verbatim}
-%     \mathrm{km}\,\mathrm{s}^{-1}
-%   \end{verbatim}
-% \end{function}
-%
-% \begin{function}{\siunitx_unit_format:nNN}
-%   \begin{syntax}
-%     \cs{siunitx_unit_format:nNN} \Arg{units} \meta{tl~var} \meta{fp~var}
-%   \end{syntax}
-%   This function formats the \meta{units} in the same way as described for
-%   \cs{siunitx_unit_format:nN}. When the input is given in symbolic form,
-%   any decimal unit prefixes will be extracted and the overall power of
-%   ten that these represent will be stored in the \meta{fp~var}.
-%
-%   For example,
-%   \begin{verbatim}
-%     \siunitx_unit_format:nNN { \kilo \metre \per \second }
-%       \l_tmpa_tl \l_tmpa_fp
-%   \end{verbatim}
-%   will, with standard settings, result in \cs{l_tmpa_tl} being set to
-%   \begin{verbatim}
-%     \mathrm{m}\,\mathrm{s}^{-1}
-%   \end{verbatim}
-%   with \cs{l_tmpa_fp} taking value~$3$. Note that the latter is a floating
-%   point variable: it is possible for non-integer values to be obtained here.
-% \end{function}
-%
-% \section{Defining symbolic units}
-%
-% \begin{function}{\siunitx_declare_prefix:Nnn}
-%   \begin{syntax}
-%     \cs{siunitx_declare_prefix:Nnn} \meta{prefix} \Arg{symbol} \Arg{power}
-%   \end{syntax}
-%   Defines a symbolic \meta{prefix} (which should be a control sequence
-%   such as |\kilo|) to be converted by the parser to the \meta{symbol}.
-%   The latter should consist of literal content (\emph{e.g.}~|k|).
-%   In literal mode the \meta{symbol} will be typeset directly. The prefix
-%   should represent an integer \meta{power} of $10$, and this information
-%   may be used to convert from one or more \meta{prefix} symbols to an
-%   overall power applying to a unit. See also
-%   \cs{siunitx_declare_prefix:Nn}.
-% \end{function}
-%
-% \begin{function}{\siunitx_declare_prefix:Nn}
-%   \begin{syntax}
-%     \cs{siunitx_declare_prefix:Nn}  \meta{prefix} \Arg{symbol}
-%   \end{syntax}
-%   Defines a symbolic \meta{prefix} (which should be a control sequence
-%   such as |\kilo|) to be converted by the parser to the \meta{symbol}.
-%   The latter should consist of literal content (\emph{e.g.}~|k|).
-%   In literal mode the \meta{symbol} will be typeset directly. In contrast
-%   to \cs{siunitx_declare_prefix:Nnn}, there is no assumption about the
-%   mathematical nature of the \meta{prefix}, \emph{i.e.}~the prefix may
-%   represent a power of any base. As a result, no conversion of the
-%   \meta{prefix} to a numerical power will be possible.
-% \end{function}
-%
-% \begin{function}{\siunitx_declare_power:NNn}
-%   \begin{syntax}
-%     \cs{siunitx_declare_power:NnN} \meta{pre-power} \meta{post-power} \Arg{value}
-%   \end{syntax}
-%   Defines \emph{two} symbolic \meta{powers} (which should be control
-%   sequences such as |\squared|) to be converted by the parser to the
-%   \meta{value}. The latter should be an integer or floating point number in
-%   the format defined for \pkg{l3fp}. Powers may precede a unit or be give
-%   after it: both forms are declared at once, as indicated by the argument
-%   naming. In literal mode, the \meta{value} will be applied as
-%   a superscript to either the next token in the input (for the
-%   \meta{pre-power}) or appended to the previously-typeset material
-%   (for the \meta{post-power}).
-% \end{function}
-%
-% \begin{function}{\siunitx_declare_qualifier:Nn}
-%   \begin{syntax}
-%     \cs{siunitx_declare_qualifier:Nn} \meta{qualifier} \Arg{meaning}
-%   \end{syntax}
-%   Defines a symbolic \meta{qualifier} (which should be a control sequence
-%   such as |\catalyst|) to be converted by the parser to the \meta{meaning}.
-%   The latter should consist of literal content (\emph{e.g.}~|cat|). In
-%   literal mode the \meta{meaning} will be typeset following a space after
-%   the unit to which it applies.
-% \end{function}
-%
-% \begin{function}{\siunitx_declare_unit:Nn, \siunitx_declare_unit:Nx}
-%   \begin{syntax}
-%     \cs{siunitx_declare_unit:Nn} \meta{unit} \Arg{meaning}
-%   \end{syntax}
-%   Defines a symbolic \meta{unit} (which should be a control sequence
-%   such as |\kilogram|) to be converted by the parser to the \meta{meaning}.
-%   The latter may consist of literal content (\emph{e.g.}~|kg|), other
-%   symbolic unit commands (\emph{e.g.}~|\kilo\gram|) or a mixture of the two.
-%   In literal mode the \meta{meaning} will be typeset directly.
-% \end{function}
-%
-% \begin{variable}{\l_siunitx_unit_symbolic_seq}
-%   This sequence contains all of the symbolic \meta{unit} names defined :
-%   these will be in the form of control sequences such as |\kilogram|.
-%   The order of the sequence is unimportant.
-% \end{variable}
-%
-% \section{Pre-defined symbolic unit components}
-%
-% The unit parser is defined to recognise a number of pre-defined units,
-% prefixes and powers, and also interpret a small selection of
-% \enquote{generic} symbolic parts.
-%
-% Broadly, the pre-defined units are those defined by the \textsc{bipm} in the
-% documentation for the \emph{International System of Units} (SI)~\cite{BIPM}.
-% As far as possible, the names given to the command names for units are those
-% used by the \textsc{bipm}, omitting spaces and using only \textsc{ascii}
-% characters. The standard symbols are also taken from the same documentation.
-% In the following documentation, the order of the description of units
-% broadly follows the SI~Brochure.
-%
-% \begin{function}
-%   {
-%     \kilogram ,
-%     \metre    ,
-%     \meter    ,
-%     \mole     ,
-%     \kelvin   ,
-%     \candela  ,
-%     \second   ,
-%     \ampere
-%   }
-%   The base units as defined in Section~2.1 of the SI Brochure~\cite{SI:2.1}.
-%   Notice that \cs{meter} is defined as an alias for \cs{metre} as the former
-%   spelling is common in the US (although the latter is the official spelling).
-% \end{function}
-%
-% \begin{function}{\gram}
-%   The base unit \cs{kilogram} is defined using an SI prefix: as such the
-%   (derived) unit \cs{gram} is required by the module to correctly produce
-%   output for the \cs{kilogram}.
-% \end{function}
-%
-% \begin{function}
-%   {
-%     \yocto ,
-%     \zepto ,
-%     \atto  ,
-%     \femto ,
-%     \pico  ,
-%     \nano  ,
-%     \micro ,
-%     \milli ,
-%     \centi ,
-%     \deci  ,
-%     \deca  ,
-%     \deka  ,
-%     \hecto ,
-%     \kilo  ,
-%     \mega  ,
-%     \giga  ,
-%     \tera  ,
-%     \peta  ,
-%     \exa   ,
-%     \zetta ,
-%     \yotta
-%   }
-%   Prefixes, all of which are integer powers of $10$: the powers are stored
-%   internally by the module and can be used for conversion from prefixes to
-%   their numerical equivalent. These prefixes are documented in Section~3.1
-%   of the SI~Brochure~\cite{SI:3.1}.
-%
-%   Note that the \cs{kilo} prefix is required to
-%   define the base \cs{kilogram} unit. Also note the two spellings available
-%   for \cs{deca}/\cs{deka}.
-% \end{function}
-%
-% \begin{function}
-%   {
-%     \becquerel     ,
-%     \degreeCelsius ,
-%     \coulomb       ,
-%     \farad         ,
-%     \gray          ,
-%     \hertz         ,
-%     \henry         ,
-%     \joule         ,
-%     \katal         ,
-%     \lumen         ,
-%     \lux           ,
-%     \newton        ,
-%     \ohm           ,
-%     \pascal        ,
-%     \radian        ,
-%     \siemens       ,
-%     \sievert       ,
-%     \steradian     ,
-%     \tesla         ,
-%     \volt          ,
-%     \watt          ,
-%     \weber
-%   }
-%   The defined SI~units with defined names and symbols, as given in
-%   Section~2.2.2 of the SI~Brochure~\cite{SI:2.2.2}. Notice that the names
-%   of the units are lower case with the exception of \cs{degreeCelsius}, and
-%   that this unit name includes \enquote{degree}.
-% \end{function}
-%
-% \begin{function}
-%   {
-%     \day       ,
-%     \hectare   ,
-%     \hour      ,
-%     \litre     ,
-%     \liter     ,
-%     \minute    ,
-%     \tonne
-%   }
-%   Units accepted for use with the SI: here \cs{minute} is a unit of time
-%   not of plane angle. These units are taken from Table~4.1 of the
-%   SI~Brochure~\cite{SI:T6}.
-%
-%   For the unit \cs{litre}, both |l| and |L| are listed
-%   as acceptable symbols: the latter is the standard setting of the module.
-%   The alternative spelling \cs{liter} is also given for this unit for US
-%   users (as with \cs{metre}, the official spelling is \enquote{re}).
-% \end{function}
-%
-% \begin{function}
-%   {
-%     \arcminute ,
-%     \arcsecond ,
-%     \degree
-%   }
-%   Units for plane angles accepted for use with the SI: to avoid a clash
-%   with units for time, here \cs{arcminute} and \cs{arcsecond} are used in
-%   place of \cs{minute} and \cs{second}. These units are taken from Table~4.1
-%   of the SI~Brochure~\cite{SI:T6}.
-% \end{function}
-%
-% \begin{function}
-%   {
-%     \astronomicalunit ,
-%     \atomicmassunit   ,
-%     \auaction         ,
-%     \aucharge         ,
-%     \auenergy         ,
-%     \aulength         ,
-%     \aumass           ,
-%     \autime           ,
-%     \bohr             ,
-%     \dalton           ,
-%     \electronvolt     ,
-%     \hartree          ,
-%     \nuaction         ,
-%     \numass           ,
-%     \nuspeed          ,
-%     \nutime
-%   }
-%   Non-SI where values must be determined experimentally. These units are taken from
-%   Table~7 of the SI~Brochure~\cite{SI:T7}. Where no better name is given for
-%   the unit in the SI~Brochure, the prefixes |nu| (natural unit) and |au| (atomic
-%   unit) are used.
-%
-%   Note that the value of the natural unit of speed (the speed of light) is used
-%   to define the second and is thus not determined by experiment: it is however
-%   included in this set of units.
-% \end{function}
-%
-% \begin{function}
-%   {
-%     \angstrom          ,
-%     \bar               ,
-%     \barn              ,
-%     \bel               ,
-%     \decibel           ,
-%     \knot              ,
-%     \millimetremercury ,
-%     \nauticalmile      ,
-%     \neper
-%   }
-%   Non-SI units accepted for use with the SI. These units are taken from
-%   Table~8 of the SI~Brochure~\cite{SI:T8}.
-% \end{function}
-%
-% \begin{function}
-%   {
-%     \dyne    ,
-%     \erg     ,
-%     \gal     ,
-%     \gauss   ,
-%     \maxwell ,
-%     \oersted ,
-%     \phot    ,
-%     \poise   ,
-%     \stilb   ,
-%     \stokes
-%   }
-%   Non-SI units associated with the CGS and the CGS-Gaussian system of units.
-%   These units are taken from Table~9 of the SI~Brochure~\cite{SI:T9}.
-% \end{function}
-%
-% \begin{function}{\percent}
-%   The mathematical concept of percent, usable with the SI as detailed in
-%   Section~5.3.7 of the SI~Brochure~\cite{SI:5.3.7}.
-% \end{function}
-%
-% \begin{function}{\square, \cubic}
-%   \begin{syntax}
-%     \cs{square} \meta{prefix} \meta{unit}
-%     \cs{cubic} \meta{prefix} \meta{unit}
-%   \end{syntax}
-%   Pre-defined unit powers which apply to the next \meta{prefix}/\meta{unit}
-%   combination.
-% \end{function}
-%
-% \begin{function}{\squared, \cubed}
-%   \begin{syntax}
-%     \meta{prefix} \meta{unit} \cs{squared}
-%     \meta{prefix} \meta{unit} \cs{cubed}
-%   \end{syntax}
-%   Pre-defined unit powers which apply to the preceding
-%   \meta{prefix}/\meta{unit} combination.
-% \end{function}
-%
-% \begin{function}{\per}
-%   \begin{syntax}
-%     \cs{per} \meta{prefix} \meta{unit} \meta{power}
-%   \end{syntax}
-%   Indicates that the next \meta{prefix}/\meta{unit}/\meta{power} combination
-%   is reciprocal, \emph{i.e.}~raises it to the power $-1$. This symbolic
-%   representation may be applied in addition to a \cs{power}, and will work
-%   correctly if the \cs{power} itself is negative. In literal mode \cs{per}
-%   will print a slash (\enquote{$/$}).
-% \end{function}
-%
-% \begin{function}{\cancel}
-%   \begin{syntax}
-%     \cs{cancel} \meta{prefix} \meta{unit} \meta{power}
-%   \end{syntax}
-%   Indicates that the next \meta{prefix}/\meta{unit}/\meta{power} combination
-%   should be \enquote{cancelled out}. In the parsed output, the entire unit
-%   combination will be given as the argument to a function \cs{cancel}, which
-%   is assumed to be available at a higher level. In literal mode, the same
-%   higher-level \cs{cancel} will be applied to the next token. It is the
-%   responsibility of the calling code to provide an appropriate definition
-%   for \cs{cancel} outside of the scope of the unit parser.
-% \end{function}
-%
-% \begin{function}{\highlight}
-%   \begin{syntax}
-%     \cs{highlight} \Arg{color} \meta{prefix} \meta{unit} \meta{power}
-%   \end{syntax}
-%   Indicates that the next \meta{prefix}/\meta{unit}/\meta{power} combination
-%   should be highlighted in the specified \meta{color}. In the parsed output,
-%   the entire unit combination will be given as the argument to a function
-%   \cs{textcolor}, which is assumed to be available at a higher level. In
-%   literal mode, the same higher-level \cs{textcolor} will be applied to the
-%   next token. It is the responsibility of the calling code to provide an
-%   appropriate definition for \cs{textcolor} outside of the scope of the unit
-%   parser.
-% \end{function}
-%
-% \begin{function}{\of}
-%   \begin{syntax}
-%     \meta{prefix} \meta{unit} \meta{power} \cs{of} \Arg{qualifier}
-%   \end{syntax}
-%   Indicates that the \meta{qualifier} applies to the current
-%   \meta{prefix}/\meta{unit}/\meta{power} combination. In parsed mode, the
-%   display of the result will depend upon module options. In literal mode,
-%   the \meta{qualifier} will be printed in parentheses following the preceding
-%   \meta{unit} and a full-width space.
-% \end{function}
-%
-% \begin{function}{\raiseto, \tothe}
-%   \begin{syntax}
-%     \cs{raiseto} \Arg{power} \meta{prefix} \meta{unit}
-%     \meta{prefix} \meta{unit} \cs{tothe} \Arg{power}
-%   \end{syntax}
-%   Indicates that the \meta{power} applies to the current
-%   \meta{prefix}/\meta{unit} combination. As shown, \cs{raiseto} applies to
-%   the next \meta{unit} whereas \cs{tothe} applies to the preceding unit. In
-%   literal mode the \cs{power} will be printed as a superscript attached to
-%   the next token (\cs{raiseto}) or preceding token (\cs{tothe}) as
-%   appropriate.
-% \end{function}
-%
-% \subsection{Key--value options}
-%
-% The options defined by this submodule are available within the \pkg{l3keys}
-% |siunitx| tree.
-%
-% \begin{function}{bracket-denominator}
-%   \begin{syntax}
-%     |bracket-denominator| = |true|\verb"|"|false|
-%   \end{syntax}
-%   Switch to determine whether brackets are added to the denominator part of
-%   a unit when printed using inline fractional form (with |per-mode| as
-%   |repeated-symbol|, |symbol| or |symbol-or-fraction|). The standard setting
-%   is |true|.
-% \end{function}
-%
-% \begin{function}{fraction-command}
-%   \begin{syntax}
-%     |fraction-command| = \meta{command}
-%   \end{syntax}
-%   Command used to create fractional output when |per-mode| is set to
-%   |fraction|. The standard setting is |\frac|.
-% \end{function}
-%
-% \begin{function}{parse-units}
-%   \begin{syntax}
-%     |parse-units| = |true|\verb"|"|false|
-%   \end{syntax}
-%   Determines whether parsing of unit symbols is attempted or literal
-%   mode is used directly. The standard setting is |true|.
-% \end{function}
-%
-% \begin{function}{per-mode}
-%   \begin{syntax}
-%     |per-mode| = \meta{choice}
-%   \end{syntax}
-%   Selects how the negative powers (\cs{per}) are formatted: a choice from
-%   the options |fraction|, |power|, |power-positive-first|, |repeated-symbol|,
-%   |symbol| and |symbol-or-fraction|. The option |fraction| generates
-%   fractional output when appropriate using the command specified by
-%   the |fraction-command| option. The setting |power| uses reciprocal powers
-%   leaving the units in the order of input, while |power-positive-first| uses
-%   the same display format but sorts units such that the positive powers
-%   come before negative ones. The |symbol| setting uses a symbol (specified
-%   by |per-symbol|) between positive and negative powers, while
-%   |repeated-symbol| uses the same symbol but places it before \emph{every}
-%   unit with a negative power (this is mathematically \enquote{wrong} but
-%   often seen in real work). Finally, |symbol-or-fraction| acts like
-%   |symbol| for inline output and like |fraction| when the output is used
-%   in a display math environment. The standard setting is |power|.
-% \end{function}
-%
-% \begin{function}{per-symbol}
-%   \begin{syntax}
-%     |per-symbol| = \meta{symbol}
-%   \end{syntax}
-%   Specifies the symbol to be used to denote negative powers when the option
-%   |per-mode| is set to |repeated-symbol|, |symbol| or |symbol-or-fraction|.
-%   The standard setting is |/|.
-% \end{function}
-%
-% \begin{function}{qualifier-mode}
-%   \begin{syntax}
-%     |qualifier-mode| = \meta{choice}
-%   \end{syntax}
-%   Selects how qualifiers are formatted: a choice from the options |brackets|,
-%   |combine|, |phrase| and |subscript|. The option |bracket| wraps
-%   the qualifier in parenthesis, |combine| joins the qualifier with the unit
-%   directly, |phrase| inserts the content stored by the option
-%   |qualifier-phrase| between the unit and qualifier, and |subscript| formats
-%   the qualifier as a subscript. The standard setting is |subscript|.
-% \end{function}
-%
-% \begin{function}{qualifier-phrase}
-%   \begin{syntax}
-%     |qualifier-phrase| = \meta{choice}
-%   \end{syntax}
-%   Defines the text to be inserted between a unit and qualifier when
-%   |qualifier-mode| is set to |phrase|. This material is inserted without
-%   any font control and so if text mode is required it should be
-%   included in the setting, for example \verb*|\ \mbox{of}\ |. The
-%   standard setting is a full width space (\verb*|\ |).
-% \end{function}
-%
-% \begin{function}{sticky-per}
-%   \begin{syntax}
-%     |sticky-per| = |true|\verb"|"|false|
-%   \end{syntax}
-%   Used to determine whether \cs{per} should be applied one a unit-by-unit
-%   basis (when |false|) or should apply to all following units
-%   (when |true|). The latter mode is somewhat akin conceptually to the
-%   \TeX{} \cs{over} primitive. The standard setting is |false|.
-% \end{function}
-%
-% \begin{function}{unit-close-bracket}
-%   \begin{syntax}
-%     |unit-close-bracket| = \meta{symbol}
-%   \end{syntax}
-%   Bracket symbol used to close a matched pair around units when once is
-%   required to maintain mathematical logic. The standard setting is ^^A (
-%   |)|.
-% \end{function}
-%
-% \begin{function}{unit-open-bracket}
-%   \begin{syntax}
-%     |unit-open-bracket| = \meta{symbol}
-%   \end{syntax}
-%   Bracket symbol used to open a matched pair around units when once is
-%   required to maintain mathematical logic. The standard setting is |(|. ^^A )
-% \end{function}
-%
-% \begin{function}{unit-product}
-%   \begin{syntax}
-%     |unit-product| = \meta{separator}
-%   \end{syntax}
-%   Inserted between unit combinations in parsed mode, and used to replace
-%   |.| and |~| in literal mode. The standard setting is |\,|.
-% \end{function}
-%
-% \end{documentation}
-%
-% \begin{implementation}
-%
-% \section{\pkg{siunitx-unit} implementation}
-%
-% Start the \pkg{DocStrip} guards.
-%    \begin{macrocode}
-%<*package>
-%    \end{macrocode}
-%
-% Identify the internal prefix (\LaTeX3 \pkg{DocStrip} convention): only
-% internal material in this \emph{submodule} should be used directly.
-%    \begin{macrocode}
-%<@@=siunitx_unit>
-%    \end{macrocode}
-%
-% \subsection{Initial set up}
-%
-% The mechanisms defined here need a few variables to exist and to be
-% correctly set: these don't belong to one subsection and so are created
-% in a small general block.
-%
-%   Variants not provided by \pkg{expl3}.
-%    \begin{macrocode}
-\cs_generate_variant:Nn \tl_replace_all:Nnn { NnV }
-%    \end{macrocode}
-%
-% \begin{variable}{\l_@@_tmp_fp}
-% \begin{variable}{\l_@@_tmp_int}
-% \begin{variable}{\l_@@_tmp_tl}
-%   Scratch space.
-%    \begin{macrocode}
-\fp_new:N \l_@@_tmp_fp
-\int_new:N \l_@@_tmp_int
-\tl_new:N  \l_@@_tmp_tl
-%    \end{macrocode}
-% \end{variable}
-% \end{variable}
-% \end{variable}
-%
-% \begin{variable}{\c_@@_math_subscript_tl}
-%   Useful tokens with awkward category codes.
-%    \begin{macrocode}
-\tl_const:Nx \c_@@_math_subscript_tl
-  { \char_generate:nn { `\_ } { 8 } }
-%    \end{macrocode}
-% \end{variable}
-%
-% \begin{variable}{\l_@@_parsing_bool}
-%   A boolean is used to indicate when the symbolic unit functions should
-%   produce symbolic or literal output. This is used when the symbolic names
-%   are used along with literal input, and ensures that there is a sensible
-%   fall-back for these cases.
-%    \begin{macrocode}
-\bool_new:N \l_@@_parsing_bool
-%    \end{macrocode}
-% \end{variable}
-%
-% \begin{variable}{\l_@@_test_bool}
-%   A switch used to indicate that the code is testing the input to find
-%   if there is any typeset output from individual unit macros. This is needed
-%   to allow the \enquote{base} macros to be found, and also to pick up the
-%   difference between symbolic and literal unit input.
-%    \begin{macrocode}
-\bool_new:N \l_@@_test_bool
-%    \end{macrocode}
-% \end{variable}
-%
-% \begin{macro}{\@@_if_symbolic:nTF}
-%   The test for symbolic units is needed in two places. First, there is the
-%   case of \enquote{pre-parsing} input to check if it can be parsed. Second,
-%   when parsing there is a need to check if the current unit is built up
-%   from others (symbolic) or is defined in terms of some literals. To do this,
-%   the approach used is to set all of the symbolic unit commands expandable
-%   and to do nothing, with the few special cases handled manually. If
-%   an \texttt{f}-type definition then yields nothing at all then the
-%   assumption is that the input is symbolic. (We use \texttt{f}-type
-%   expansion since it will turn the symbolic unit names into nothing at
-%   all but doesn't require them to be expandable.)
-%    \begin{macrocode}
-\prg_new_protected_conditional:Npnn \@@_if_symbolic:n #1 { TF }
-  {
-    \group_begin:
-      \bool_set_true:N \l_@@_test_bool
-      \tl_set:Nf \l_@@_tmp_tl {#1}
-    \exp_args:NNV \group_end:
-    \tl_if_blank:nTF \l_@@_tmp_tl
-      { \prg_return_true: }
-      { \prg_return_false: }
-  }
-%    \end{macrocode}
-% \end{macro}
-%
-% \subsection{Defining symbolic unit}
-%
-% Unit macros and related support are created here. These exist only within
-% the scope of the unit processor code, thus not polluting document-level
-% namespace and allowing overlap with other areas in the case of useful short
-% names (for example \cs{pm}). Setting up the mechanisms to allow this requires
-% a few additional steps on top of simply saving the data given by the user
-% in creating the unit.
-%
-% \begin{variable}{\l_siunitx_unit_symbolic_seq}
-%   A list of all of the symbolic units, \emph{etc.}, set up. This is needed
-%   to allow the symbolic names to be defined within the scope of the unit
-%   parser but not elsewhere using simple mappings.
-%    \begin{macrocode}
-\seq_new:N \l_siunitx_unit_symbolic_seq
-%    \end{macrocode}
-% \end{variable}
-%
-% \begin{macro}{\@@_set_symbolic:Nnn}
-% \begin{macro}{\@@_set_symbolic:Npnn}
-% \begin{macro}{\@@_set_symbolic:NNpnn}
-%   The majority of the work for saving each symbolic definition is the same
-%   irrespective of the item being defined (unit, prefix, power, qualifier).
-%   This is therefore all carried out in a single internal function which
-%   does the common tasks. The three arguments here are the symbolic macro
-%   name, the literal output and the code to insert when doing full unit
-%   parsing. To allow for the \enquote{special cases} (where arguments are
-%   required) the entire mechanism is set up in a two-part fashion allowing
-%   for flexibility at the slight cost of additional functions.
-%
-%   Importantly, notice that the unit macros are declared as expandable. This
-%   is required so that literals can be correctly converted into a token list
-%   of material which does not depend on local redefinitions for the unit
-%   macros. That is required so that the unit formatting system can be grouped.
-%    \begin{macrocode}
-\cs_new_protected:Npn \@@_set_symbolic:Nnn #1
-  { \@@_set_symbolic:NNnnn \cs_set:cpn #1 { } }
-\cs_new_protected:Npn \@@_set_symbolic:Npnn #1#2#
-  { \@@_set_symbolic:NNnnn \cs_set:cpn #1 {#2} }
-\cs_new_protected:Npn \@@_set_symbolic:NNnnn #1#2#3#4#5
-  {
-    \seq_put_right:Nn \l_siunitx_unit_symbolic_seq {#2}
-    #1 { units ~ > ~ \token_to_str:N #2 } #3
-      {
-        \bool_if:NF \l_@@_test_bool
-          {
-            \bool_if:NTF \l_@@_parsing_bool
-              {#5}
-              {#4}
-          }
-      }
-  }
-%    \end{macrocode}
-% \end{macro}
-% \end{macro}
-% \end{macro}
-%
-% \begin{macro}{\siunitx_declare_power:NNn}
-%   Powers can come either before or after the unit. As they always come
-%   (logically) in matching, we handle this by declaring two commands,
-%   and setting each up separately.
-%    \begin{macrocode}
-\cs_new_protected:Npn \siunitx_declare_power:NNn #1#2#3
-  {
-    \@@_set_symbolic:Nnn #1
-      { \@@_literal_power:nN {#1} }
-      { \@@_parse_power:nnN {#1} {#3} \c_true_bool }
-    \@@_set_symbolic:Nnn #2
-      { ^ {#3} }
-      { \@@_parse_power:nnN {#2} {#3} \c_false_bool }
-  }
-%    \end{macrocode}
-% \end{macro}
-%
-% \begin{macro}{\siunitx_declare_prefix:Nn}
-% \begin{macro}{\siunitx_declare_prefix:Nnn}
-% \begin{variable}
-%   {\l_@@_prefixes_forward_prop, \l_@@_prefixes_reverse_prop}
-%   For prefixes there are a couple of options. In all cases, the basic
-%   requirement is to set up to parse the prefix using the appropriate
-%   internal function. For prefixes which are powers of $10$, there is also
-%   the need to be able to do conversion to/from the numerical equivalent.
-%   That is handled using two properly lists which can be used to supply
-%   the conversion data later.
-%    \begin{macrocode}
-\cs_new_protected:Npn \siunitx_declare_prefix:Nn #1#2
-  {
-    \@@_set_symbolic:Nnn #1
-      {#2}
-      { \@@_parse_prefix:Nn #1 {#2} }
-  }
-\cs_new_protected:Npn \siunitx_declare_prefix:Nnn #1#2#3
-  {
-    \siunitx_declare_prefix:Nn #1 {#2}
-    \prop_put:Nnn \l_@@_prefixes_forward_prop {#2} {#3}
-    \prop_put:Nnn \l_@@_prefixes_reverse_prop {#3} {#2}
-  }
-\prop_new:N \l_@@_prefixes_forward_prop
-\prop_new:N \l_@@_prefixes_reverse_prop
-%    \end{macrocode}
-% \end{variable}
-% \end{macro}
-% \end{macro}
-%
-% \begin{macro}{\siunitx_declare_qualifier:Nn}
-%   Qualifiers are relatively easy to handle: nothing to do other than save
-%   the input appropriately.
-%    \begin{macrocode}
-\cs_new_protected:Npn \siunitx_declare_qualifier:Nn #1#2
-  {
-    \@@_set_symbolic:Nnn #1
-      { \  ( #2 ) }
-      { \@@_parse_qualifier:nn {#1} {#2} }
-  }
-%    \end{macrocode}
-% \end{macro}
-%
-% \begin{macro}{\siunitx_declare_unit:Nn, \siunitx_declare_unit:Nx}
-%   For the unit parsing, allowing for variations in definition order requires
-%   that a test is made for the output of each unit at point of use.
-%    \begin{macrocode}
-\cs_new_protected:Npn \siunitx_declare_unit:Nn #1#2
-  {
-    \@@_set_symbolic:Nnn #1
-      {#2}
-      {
-        \@@_if_symbolic:nTF {#2}
-          {#2}
-          { \@@_parse_unit:Nn #1 {#2} }
-      }
-  }
-\cs_generate_variant:Nn \siunitx_declare_unit:Nn { Nx }
-%    \end{macrocode}
-% \end{macro}
-%
-% \subsection{Non-standard symbolic units}
-%
-% A few of the symbolic units require non-standard definitions: these are
-% created here. They all use parts of the more general code but have particular
-% requirements which can only be addressed by hand. Some of these could in
-% principle be used in place of the dedicated definitions above, but at point
-% of use that would then require additional expansions for each unit parsed:
-% as the macro names would still be needed, this does not offer any real
-% benefits.
-%
-% \begin{macro}{\per}
-%   The \cs{per} symbolic unit is a bit special: it has a mechanism entirely
-%   different from everything else, so has to be set up by hand. In literal
-%   mode it is represented by a very simple symbol!
-%    \begin{macrocode}
-\@@_set_symbolic:Nnn \per
-  { / }
-  { \@@_parse_per: }
-%    \end{macrocode}
-% \end{macro}
-%
-% \begin{macro}{\cancel}
-% \begin{macro}{\highlight}
-%   The two special cases, \cs{cancel} and \cs{highlight}, are easy to deal
-%   with when parsing. When not parsing, a precaution is taken to ensure that
-%   the user level equivalents always get a braced argument.
-%    \begin{macrocode}
-\@@_set_symbolic:Npnn \cancel
-  { \@@_literal_special:nN { \cancel } }
-  { \@@_parse_special:n { \cancel } }
-\@@_set_symbolic:Npnn \highlight #1
-  { \@@_literal_special:nN { \textcolor {#1} } }
-  { \@@_parse_special:n { \textcolor {#1} } }
-%    \end{macrocode}
-% \end{macro}
-% \end{macro}
-%
-% \begin{macro}{\of}
-%   The generic qualifier is simply the same as the dedicated ones except for
-%   needing to grab an argument.
-%    \begin{macrocode}
-\@@_set_symbolic:Npnn \of #1
-  { \  ( #1 ) }
-  { \@@_parse_qualifier:nn { \of {#1} } {#1} }
-%    \end{macrocode}
-% \end{macro}
-%
-% \begin{macro}{\raiseto, \tothe}
-%   Generic versions of the pre-defined power macros. These require an
-%   argument and so cannot be handled using the general approach. Other than
-%   that, the code here is very similar to that in
-%   \cs{siunitx_unit_power_set:NnN}.
-%    \begin{macrocode}
-\@@_set_symbolic:Npnn \raiseto #1
-  { \@@_literal_power:nN {#1} }
-  { \@@_parse_power:nnN { \raiseto {#1} } {#1} \c_true_bool }
-\@@_set_symbolic:Npnn \tothe #1
-  { ^ {#1} }
-  { \@@_parse_power:nnN { \tothe {#1} } {#1} \c_false_bool }
-%    \end{macrocode}
-% \end{macro}
-%
-% \subsection{Main formatting routine}
-%
-% Unit input can take two forms, \enquote{literal} units (material to be
-% typeset directly) or \enquote{symbolic} units (macro-based). Before any
-% parsing or typesetting is carried out, a small amount of pre-parsing has to
-% be carried out to decide which of these cases applies.
-%
-% \begin{variable}{\l_@@_product_tl}
-%   Options which apply to the main formatting routine, and so are not tied
-%   to either symbolic or literal input.
-%    \begin{macrocode}
-\keys_define:nn { siunitx }
-  {
-    unit-product .tl_set:N = \l_@@_product_tl
-  }
-%    \end{macrocode}
-% \end{variable}
-%
-% \begin{variable}{\l_@@_formatted_tl}
-%   A token list for the final formatted result: may or may not be generated
-%   by the parser, depending on the nature of the input.
-%    \begin{macrocode}
-\tl_new:N \l_@@_formatted_tl
-%    \end{macrocode}
-% \end{variable}
-%
-% \begin{macro}{\siunitx_unit_format:nN}
-% \begin{macro}{\siunitx_unit_format:nNN}
-% \begin{macro}{\@@_format:nNN}
-% \begin{macro}{\@@_format_aux:}
-%   Formatting parsed units can take place either with the prefixes printed or
-%   separated out into a power of ten. This variation is handled using two
-%   separate functions: as this submodule does not really deal with numbers,
-%   formatting the numeral part here would be tricky and it is better therefore
-%   to have a mechanism to return a simple numerical power. At the same time,
-%   most uses will no want this more complex return format and so a version of
-%   the code which does not do this is also provided.
-%
-%   The main unit formatting routine groups all of the parsing/formatting, so
-%   that the only value altered will be the return token list. As definitions
-%   for the various unit macros are not globally created, the first step is to
-%   map over the list of names and active the unit definitions: these do
-%   different things depending on the switches set. There is then a decision to
-%   be made: is the unit input one that can be parsed (\enquote{symbolic}), or
-%   is is one containing one or more literals. In the latter case, there is a
-%   still the need to convert the input into an expanded token list as some
-%   parts of the input could still be using unit macros.
-%
-%   Notice that for \cs{siunitx_unit_format:nN} a second return value from the
-%   auxiliary has to be allowed for, but is simply discarded.
-%    \begin{macrocode}
-\cs_new_protected:Npn \siunitx_unit_format:nN #1#2
-  {
-    \bool_set_false:N \l_@@_prefix_power_bool
-    \@@_format:nNN {#1} #2 \l_@@_tmp_fp
-  }
-\cs_new_protected:Npn \siunitx_unit_format:nNN #1#2#3
-  {
-    \bool_set_true:N \l_@@_prefix_power_bool
-    \@@_format:nNN {#1} #2 #3
-  }
-\cs_new_protected:Npn \@@_format:nNN #1#2#3
-  {
-    \group_begin:
-      \seq_map_inline:Nn \l_siunitx_unit_symbolic_seq
-        { \cs_set_eq:Nc ##1 { units ~ > ~ \token_to_str:N ##1 } }
-      \tl_clear:N \l_@@_formatted_tl
-      \fp_zero:N \l_@@_prefix_fp
-      \bool_if:NTF \l_@@_parse_bool
-        {
-          \@@_if_symbolic:nTF {#1}
-            {
-              \@@_parse:n {#1}
-              \prop_if_empty:NF \l_@@_parsed_prop
-                { \@@_format_parsed: }
-            }
-            { \@@_format_literal:n {#1} }
-        }
-        { \@@_format_literal:n {#1} }
-      \cs_set_protected:Npx \@@_format_aux:
-        {
-          \tl_set:Nn \exp_not:N #2
-            { \exp_not:V \l_@@_formatted_tl }
-          \fp_set:Nn \exp_not:N #3
-            { \fp_use:N \l_@@_prefix_fp }
-        }
-    \exp_after:wN \group_end:
-    \@@_format_aux:
-  }
-\cs_new_protected:Npn \@@_format_aux: { }
-%    \end{macrocode}
-% \end{macro}
-% \end{macro}
-% \end{macro}
-% \end{macro}
-%
-% \subsection{Formatting literal units}
-%
-% While in literal mode no parsing occurs, there is a need to provide a few
-% auxiliary functions to handle one or two special cases.
-%
-% \begin{macro}{\@@_literal_power:nN}
-%   For printing literal units which are given before the unit they apply to,
-%   there is a slight rearrangement.
-%    \begin{macrocode}
-\cs_new_protected:Npn \@@_literal_power:nN #1#2 { #2 ^ {#1} }
-%    \end{macrocode}
-% \end{macro}
-%
-% \begin{macro}{\@@_literal_special:nN}
-%   When dealing with the special cases, there is an argument to absorb. This
-%   should be braced to be passed up to the user level, which is dealt with
-%   here.
-%    \begin{macrocode}
-\cs_new_protected:Npn \@@_literal_special:nN #1#2 { #1 {#2} }
-%    \end{macrocode}
-% \end{macro}
-%
-% \begin{macro}{\@@_format_literal:n}
-% \begin{macro}{\@@_format_literal_auxi:w}
-% \begin{macro}{\@@_format_literal_auxii:w}
-% \begin{macro}{\@@_format_literal_auxiii:w}
-% \begin{macro}{\@@_format_literal_auxiv:w}
-% \begin{macro}{\@@_format_literal_auxv:w}
-% \begin{variable}{\l_@@_separator_tl}
-%   To format literal units, there are two tasks to do. The input is
-%   \texttt{x}-type expanded to force any symbolic units to be converted into
-%   their literal representation: this requires setting the appropriate
-%   switch. In the resulting token list, all |.| and |~| tokens are then
-%   replaced by the current unit product token list. To enable this to happen
-%   correctly with a normal (active) |~|, a small amount of
-%   \enquote{protection} is needed first.
-%
-%   As with other code dealing with user input, \cs{protected@edef} is used
-%   here rather than \cs{tl_set:Nx} as \LaTeXe{} robust commands may be
-%   present.
-%    \begin{macrocode}
-\group_begin:
-  \char_set_catcode_active:n { `\~ }
-  \cs_new_protected:Npx \@@_format_literal:n #1
-    {
-      \group_begin:
-        \exp_not:n { \bool_set_false:N \l_@@_parsing_bool }
-        \tl_set:Nn \exp_not:N \l_@@_tmp_tl {#1}
-        \tl_replace_all:Nnn \exp_not:N \l_@@_tmp_tl
-          { \exp_not:N ~ } { . }
-        \tl_replace_all:Nnn \exp_not:N \l_@@_tmp_tl
-          { \token_to_str:N ^ } { ^ }
-        \tl_replace_all:Nnn \exp_not:N \l_@@_tmp_tl
-          { \token_to_str:N _ } { \c_@@_math_subscript_tl }
-        \exp_not:n
-          {
-              \protected@edef \l_@@_tmp_tl
-                { \l_@@_tmp_tl }
-              \tl_clear:N \l_@@_formatted_tl
-              \tl_if_empty:NF \l_@@_tmp_tl
-                {
-                  \exp_after:wN \@@_format_literal_auxi:w
-                    \l_@@_tmp_tl .
-                    \q_recursion_tail . \q_recursion_stop
-                }
-            \exp_args:NNNV \group_end:
-            \tl_set:Nn \l_@@_formatted_tl
-              \l_@@_formatted_tl
-          }
-    }
-\group_end:
-%    \end{macrocode}
-%   To introduce the font changing commands while still allowing for line
-%   breaks in literal units, a loop is needed to replace one |.| at a time.
-%   To also allow for division, a second loop is used within that to handle
-%   |/|: as a result, the separator between parts has to be tracked.
-%    \begin{macrocode}
-\cs_new_protected:Npn \@@_format_literal_auxi:w #1 .
-  {
-    \quark_if_recursion_tail_stop:n {#1}
-    \@@_format_literal_auxii:n {#1}
-    \tl_set_eq:NN \l_@@_separator_tl \l_@@_product_tl
-    \@@_format_literal_auxi:w
-  }
-\cs_set_protected:Npn \@@_format_literal_auxii:n #1
-  {
-    \@@_format_literal_auxiii:w
-      #1 / \q_recursion_tail / \q_recursion_stop
-  }
-\cs_new_protected:Npn \@@_format_literal_auxiii:w #1 /
-  {
-    \quark_if_recursion_tail_stop:n {#1}
-    \@@_format_literal_auxiv:w #1 ^ ^ \q_stop
-    \tl_set:Nn \l_@@_separator_tl { / }
-    \@@_format_literal_auxiii:w
-  }
-%    \end{macrocode}
-%   Within each unit any sub- and superscript parts need to be separated out:
-%   wrapping everything in the font command is incorrect. The superscript part
-%   is relatively easy as there is no catcode issue or font command to add,
-%   while the subscript part needs a bit more work. As the user might have the
-%   two parts in either order, picking up the subscript needs to look in two
-%   places. We assume that only one is given: odd input here is simply accepted.
-%    \begin{macrocode}
-\use:x
-  {
-    \cs_new_protected:Npn \exp_not:N \@@_format_literal_auxiv:w
-      ##1 ^ ##2 ^ ##3 \exp_not:N \q_stop
-      {
-        \exp_not:N \@@_format_literal_auxv:w
-          ##1
-          \c_@@_math_subscript_tl
-          \c_@@_math_subscript_tl
-          \exp_not:N \q_mark
-          ##2
-          \c_@@_math_subscript_tl
-          \c_@@_math_subscript_tl
-          \exp_not:N \q_stop
-      }
-    \cs_new_protected:Npn \exp_not:N \@@_format_literal_auxv:w
-      ##1 \c_@@_math_subscript_tl
-      ##2 \c_@@_math_subscript_tl ##3
-      \exp_not:N \q_mark
-      ##4 \c_@@_math_subscript_tl
-      ##5 \c_@@_math_subscript_tl ##6
-      \exp_not:N \q_stop
-      {
-        \tl_set:Nx \exp_not:N \l_@@_formatted_tl
-          {
-            \exp_not:N \exp_not:V
-              \exp_not:N \l_@@_formatted_tl
-            \exp_not:N \tl_if_empty:NF
-              \exp_not:N \l_@@_formatted_tl
-              {
-                \exp_not:N \exp_not:V
-                  \exp_not:N \l_@@_separator_tl
-              }
-            \exp_not:N \tl_if_blank:nF {##1}
-              {
-                \exp_not:N \exp_not:N
-                  \exp_not:N \mathrm
-                  { \exp_not:N \exp_not:n {##1} }
-              }
-            \exp_not:N \tl_if_blank:nF {##4}
-              { ^ { \exp_not:N \exp_not:n {##4} } }
-            \exp_not:N \tl_if_blank:nF {##2##5}
-              {
-                \c_@@_math_subscript_tl
-                  {
-                    \exp_not:N \exp_not:N
-                      \exp_not:N \mathrm
-                      { \exp_not:N \exp_not:n {##2##5} }
-                  }
-              }
-          }
-      }
-  }
-\tl_new:N \l_@@_separator_tl
-%    \end{macrocode}
-% \end{variable}
-% \end{macro}
-% \end{macro}
-% \end{macro}
-% \end{macro}
-% \end{macro}
-% \end{macro}
-%
-% \subsection{Parsing symbolic units}
-%
-% Parsing units takes place by storing information about each unit in a
-% \texttt{prop}. As well as the unit itself, there are various other optional
-% data points, for example a prefix or a power. Some of these can come before
-% the unit, others only after. The parser therefore tracks the number of units
-% read and uses the current position to allocate data to individual units.
-%
-% The result of parsing is a property list (\cs{l_@@_parsed_prop}) which
-% contains one or more entries for each unit:
-% \begin{itemize}
-%   \item \texttt{prefix-$n$} The symbol for the prefix which applies to this
-%     unit, \emph{e.g.} for \cs{kilo} with (almost certainly) would be
-%     |k|.
-%   \item \texttt{unit-$n$} The symbol for the unit itself, \emph{e.g.}~for
-%     \cs{metre} with (almost certainly) would be |m|.
-%   \item \texttt{power-$n$} The power which a unit is raised to. During
-%     initial parsing this will (almost certainly) be positive, but is combined
-%     with \texttt{per-$n$} to give a \enquote{fully qualified} power before
-%     any formatting takes place
-%   \item \texttt{per-$n$} Indicates that \texttt{per} applies to the current
-%     unit: stored during initial parsing then combined with \texttt{power-$n$}
-%     (and removed from the list) before further work.
-%   \item \texttt{qualifier-$n$} Any qualifier which applies to the current
-%     unit.
-%   \item \texttt{special-$n$} Any \enquote{special effect} to apply to the
-%     current unit.
-% \end{itemize}
-%
-% \begin{variable}{\l_@@_sticky_per_bool}
-%   There is one option when \emph{parsing} the input (as opposed to
-%   \emph{formatting} for output): how to deal with \cs{per}.
-%    \begin{macrocode}
-\keys_define:nn { siunitx }
-  {
-    sticky-per .bool_set:N = \l_@@_sticky_per_bool
-  }
-%    \end{macrocode}
-% \end{variable}
-%
-% \begin{variable}{\l_@@_parsed_prop}
-% \begin{variable}{\l_@@_per_bool}
-% \begin{variable}{\l_@@_position_int}
-%   Parsing units requires a small number of variables are available: a
-%   \texttt{prop} for the parsed units themselves, a \texttt{bool} to
-%   indicate if \cs{per} is active and an \texttt{int} to track how many units
-%   have be parsed.
-%    \begin{macrocode}
-\prop_new:N \l_@@_parsed_prop
-\bool_new:N \l_@@_per_bool
-\int_new:N \l_@@_position_int
-%    \end{macrocode}
-% \end{variable}
-% \end{variable}
-% \end{variable}
-%
-% \begin{macro}{\@@_parse:n}
-%   The main parsing function is quite simple. After initialising the variables,
-%   each symbolic unit is set up. The input is then simply inserted into the
-%   input stream: the symbolic units themselves then do the real work of
-%   placing data into the parsing system. There is then a bit of tidying up to
-%   ensure that later stages can rely on the nature of the data here.
-%    \begin{macrocode}
-\cs_new_protected:Npn \@@_parse:n #1
-  {
-    \prop_clear:N \l_@@_parsed_prop
-    \bool_set_true:N \l_@@_parsing_bool
-    \bool_set_false:N \l_@@_per_bool
-    \bool_set_false:N \l_@@_test_bool
-    \int_zero:N \l_@@_position_int
-    #1
-    \int_step_inline:nnnn 1 1 \l_@@_position_int
-      { \@@_parse_finalise:n {##1} }
-    \@@_parse_finalise:
-  }
-%    \end{macrocode}
-% \end{macro}
-%
-% \begin{macro}{\@@_parse_add:nnnn}
-%   In all cases, storing a data item requires setting a temporary \texttt{tl}
-%   which will be used as the key, then using this to store the value. The
-%   \texttt{tl} is  set using \texttt{x}-type expansion as this will expand the
-%   unit index and any additional calculations made for this.
-%    \begin{macrocode}
-\cs_new_protected:Npn \@@_parse_add:nnnn #1#2#3#4
-  {
-    \tl_set:Nx \l_@@_tmp_tl { #1 - #2 }
-    \prop_if_in:NVTF \l_@@_parsed_prop
-      \l_@@_tmp_tl
-      {
-        \msg_error:nnxx { siunitx } { unit / duplicate-part }
-          { \exp_not:n {#1} } { \token_to_str:N #3 }
-      }
-      {
-        \prop_put:NVn \l_@@_parsed_prop
-          \l_@@_tmp_tl {#4}
-      }
-  }
-%    \end{macrocode}
-% \end{macro}
-%
-% \begin{macro}{\@@_parse_prefix:Nn}
-% \begin{macro}{\@@_parse_power:nnN}
-% \begin{macro}{\@@_parse_qualifier:nn}
-% \begin{macro}{\@@_parse_special:n}
-%   Storage of the various optional items follows broadly the same pattern
-%   in each case. The data to be stored is passed along with an appropriate
-%   key name to the underlying storage system. The details for each type of
-%   item should be relatively clear. For example, prefixes have to come before
-%   their \enquote{parent} unit and so there is some adjustment to do to add
-%   them to the correct unit.
-%    \begin{macrocode}
-\cs_new_protected:Npn \@@_parse_prefix:Nn #1#2
-  {
-    \int_set:Nn \l_@@_tmp_int { \l_@@_position_int + 1 }
-    \@@_parse_add:nnnn { prefix }
-      { \int_use:N \l_@@_tmp_int } {#1} {#2}
-  }
-\cs_new_protected:Npn \@@_parse_power:nnN #1#2#3
-  {
-    \tl_set:Nx \l_@@_tmp_tl
-      { unit- \int_use:N \l_@@_position_int }
-    \bool_lazy_or:nnTF
-      {#3}
-      {
-        \prop_if_in_p:NV
-          \l_@@_parsed_prop \l_@@_tmp_tl
-      }
-      {
-        \@@_parse_add:nnnn { power }
-          {
-            \int_eval:n
-              { \l_@@_position_int \bool_if:NT #3 { + 1 } }
-          }
-          {#1} {#2}
-      }
-      {
-        \msg_error:nnxx { siunitx }
-          { unit / part-before-unit } { power } { \token_to_str:N #1 }
-      }
-  }
-\cs_new_protected:Npn \@@_parse_qualifier:nn #1#2
-  {
-    \tl_set:Nx \l_@@_tmp_tl
-      { unit- \int_use:N \l_@@_position_int }
-    \prop_if_in:NVTF \l_@@_parsed_prop \l_@@_tmp_tl
-      {
-        \@@_parse_add:nnnn { qualifier }
-          { \int_use:N \l_@@_position_int } {#1} {#2}
-      }
-      {
-        \msg_error:nnnn { siunitx }
-          { unit / part-before-unit } { qualifier } { \token_to_str:N #1 }
-      }
-  }
-%    \end{macrocode}
-%  Special (exceptional) items should always come before the relevant units.
-%    \begin{macrocode}
-\cs_new_protected:Npn \@@_parse_special:n #1
-  {
-    \@@_parse_add:nnnn { special }
-      { \int_eval:n { \l_@@_position_int + 1 } }
-      {#1} {#1}
-  }
-%    \end{macrocode}
-% \end{macro}
-% \end{macro}
-% \end{macro}
-% \end{macro}
-%
-% \begin{macro}{\@@_parse_unit:Nn}
-%   Parsing units is slightly more involved than the other cases: this is the
-%   one place where the tracking value is incremented.  If the switch
-%   \cs{l_@@_per_bool} is set true then the current unit is also
-%   reciprocal: this can only happen if \cs{l_@@_sticky_per_bool} is also
-%   true, so only one test is required.
-%    \begin{macrocode}
-\cs_new_protected:Npn \@@_parse_unit:Nn #1#2
-  {
-    \int_incr:N \l_@@_position_int
-    \@@_parse_add:nnnn { unit }
-      { \int_use:N \l_@@_position_int }
-      {#1} {#2}
-    \bool_if:NT \l_@@_per_bool
-      {
-        \@@_parse_add:nnnn { per }
-          { \int_use:N \l_@@_position_int }
-          { \per } { true }
-      }
-  }
-%    \end{macrocode}
-% \end{macro}
-%
-% \begin{macro}{\@@_parse_per:}
-%   Storing the \cs{per} command requires adding a data item separate from
-%   the power which applies: this makes later formatting much more
-%   straight-forward. This data could in principle be combined with the
-%   \texttt{power}, but depending on the output format required that may make
-%   life  more complex. Thus this information is stored separately for later
-%   retrieval. If \cs{per} is set to be \enquote{sticky} then after parsing
-%   the first occurrence, any further uses are in error.
-%    \begin{macrocode}
-\cs_new_protected:Npn \@@_parse_per:
-  {
-    \bool_if:NTF \l_@@_sticky_per_bool
-      {
-        \bool_set_true:N \l_@@_per_bool
-        \cs_set_protected:Npn \per
-          { \msg_error:nn { siunitx } { unit / duplicate-sticky-per } }
-      }
-      {
-        \@@_parse_add:nnnn
-          { per } { \int_eval:n { \l_@@_position_int + 1 } }
-          { \per } { true }
-      }
-  }
-%    \end{macrocode}
-% \end{macro}
-%
-% \begin{macro}{\@@_parse_finalise:n}
-%   If \cs{per} applies to the current unit, the power needs to be multiplied
-%   by $-1$. That is done using an \texttt{fp} operation so that non-integer
-%   powers are supported. The flag for \cs{per} is also removed as this means
-%   we don't have to check that the original power was positive. To be on
-%   the safe side, there is a check for a trivial power at this stage.
-%    \begin{macrocode}
-\cs_new_protected:Npn \@@_parse_finalise:n #1
-  {
-    \tl_set:Nx \l_@@_tmp_tl { per- #1 }
-    \prop_if_in:NVT \l_@@_parsed_prop \l_@@_tmp_tl
-      {
-        \prop_remove:NV \l_@@_parsed_prop
-          \l_@@_tmp_tl
-        \tl_set:Nx \l_@@_tmp_tl { power- #1 }
-        \prop_get:NVNTF
-          \l_@@_parsed_prop
-          \l_@@_tmp_tl
-          \l_@@_part_tl
-          {
-            \tl_set:Nx \l_@@_part_tl
-              { \fp_eval:n { \l_@@_part_tl * -1 } }
-            \fp_compare:nNnTF \l_@@_part_tl = 1
-              {
-                \prop_remove:NV \l_@@_parsed_prop
-                  \l_@@_tmp_tl
-              }
-              {
-                \prop_put:NVV \l_@@_parsed_prop
-                  \l_@@_tmp_tl \l_@@_part_tl
-              }
-          }
-          {
-            \prop_put:NVn \l_@@_parsed_prop
-              \l_@@_tmp_tl { -1 }
-          }
-      }
-  }
-%    \end{macrocode}
-% \end{macro}
-%
-% \begin{macro}{\@@_parse_finalise:}
-%   The final task is to check that there is not a \enquote{dangling} power
-%   or prefix: these are added to the \enquote{next} unit so are easy to
-%   test for.
-%    \begin{macrocode}
-\cs_new_protected:Npn \@@_parse_finalise:
-  {
-    \clist_map_inline:nn { per , power , prefix }
-      {
-        \tl_set:Nx \l_@@_tmp_tl
-          { ##1 - \int_eval:n { \l_@@_position_int + 1 } }
-        \prop_if_in:NVT \l_@@_parsed_prop \l_@@_tmp_tl
-          { \msg_error:nnn { siunitx } { unit / dangling-part } { ##1 } }
-      }
-  }
-%    \end{macrocode}
-% \end{macro}
-%
-% \subsection{Formatting parsed units}
-%
-% \begin{variable}
-%   {
-%     \l_@@_denominator_bracket_bool ,
-%     \l_@@_fraction_function_tl     ,
-%     \l_@@_bracket_close_tl         ,
-%     \l_@@_bracket_open_tl          ,
-%     \l_@@_parse_bool               ,
-%     \l_@@_per_symbol_tl            ,
-%     \l_@@_qualifier_mode_tl        ,
-%     \l_@@_qualifier_phrase_tl
-%   }
-%   Set up the options which apply to formatting.
-%    \begin{macrocode}
-\keys_define:nn { siunitx }
-  {
-    bracket-denominator .bool_set:N =
-      \l_@@_denominator_bracket_bool ,
-    fraction-command .tl_set:N =
-      \l_@@_fraction_function_tl ,
-    parse-units .bool_set:N =
-      \l_@@_parse_bool ,
-    per-mode .choice: ,
-    per-mode / fraction .code:n =
-      {
-        \bool_set_false:N \l_@@_autofrac_bool
-        \bool_set_false:N \l_@@_per_symbol_bool
-        \bool_set_true:N  \l_@@_powers_positive_bool
-        \bool_set_true:N  \l_@@_two_part_bool
-      } ,
-    per-mode / power .code:n =
-      {
-        \bool_set_false:N \l_@@_autofrac_bool
-        \bool_set_false:N \l_@@_per_symbol_bool
-        \bool_set_false:N \l_@@_powers_positive_bool
-        \bool_set_false:N \l_@@_two_part_bool
-      } ,
-    per-mode / power-positive-first .code:n =
-      {
-        \bool_set_false:N \l_@@_autofrac_bool
-        \bool_set_false:N \l_@@_per_symbol_bool
-        \bool_set_false:N \l_@@_powers_positive_bool
-        \bool_set_true:N  \l_@@_two_part_bool
-      } ,
-    per-mode / repeated-symbol .code:n =
-      {
-        \bool_set_false:N \l_@@_autofrac_bool
-        \bool_set_true:N  \l_@@_per_symbol_bool
-        \bool_set_true:N  \l_@@_powers_positive_bool
-        \bool_set_false:N \l_@@_two_part_bool
-      } ,
-    per-mode / symbol .code:n =
-      {
-        \bool_set_false:N \l_@@_autofrac_bool
-        \bool_set_true:N  \l_@@_per_symbol_bool
-        \bool_set_true:N  \l_@@_powers_positive_bool
-        \bool_set_true:N  \l_@@_two_part_bool
-      } ,
-    per-mode / symbol-or-fraction .code:n =
-      {
-        \bool_set_true:N \l_@@_autofrac_bool
-        \bool_set_true:N \l_@@_per_symbol_bool
-        \bool_set_true:N \l_@@_powers_positive_bool
-        \bool_set_true:N \l_@@_two_part_bool
-      } ,
-    per-symbol .tl_set:N =
-      \l_@@_per_symbol_tl ,
-    qualifier-mode .choice: ,
-    qualifier-mode / bracket .code:n =
-      { \tl_set:Nn \l_@@_qualifier_mode_tl { bracket  } }  ,
-    qualifier-mode / combine .code:n =
-      { \tl_set:Nn \l_@@_qualifier_mode_tl { combine } }   ,
-    qualifier-mode / phrase .code:n =
-      { \tl_set:Nn \l_@@_qualifier_mode_tl { phrase } }    ,
-    qualifier-mode / subscript .code:n =
-      { \tl_set:Nn \l_@@_qualifier_mode_tl { subscript } } ,
-    qualifier-phrase .tl_set:N =
-      \l_@@_qualifier_phrase_tl ,
-    unit-close-bracket .tl_set:N =
-      \l_@@_bracket_close_tl ,
-    unit-open-bracket .tl_set:N =
-      \l_@@_bracket_open_tl
-  }
-%    \end{macrocode}
-% \end{variable}
-%
-% \begin{variable}{\l_@@_bracket_bool}
-%   A flag to indicate that the unit currently under construction will require
-%   brackets if a power is added.
-%    \begin{macrocode}
-\bool_new:N \l_@@_bracket_bool
-%    \end{macrocode}
-% \end{variable}
-%
-% \begin{variable}{\l_@@_font_bool}
-%   A flag to control when font wrapping is applied to the output.
-%    \begin{macrocode}
-\bool_new:N \l_@@_font_bool
-%    \end{macrocode}
-% \end{variable}
-%
-% \begin{variable}
-%   {
-%     \l_@@_autofrac_bool        ,
-%     \l_@@_powers_positive_bool ,
-%     \l_@@_per_symbol_bool      ,
-%     \l_@@_two_part_bool
-%   }
-%   Dealing with the various ways that reciprocal (\cs{per}) can be handled
-%   requires a few different switches.
-%    \begin{macrocode}
-\bool_new:N \l_@@_autofrac_bool
-\bool_new:N \l_@@_per_symbol_bool
-\bool_new:N \l_@@_powers_positive_bool
-\bool_new:N \l_@@_two_part_bool
-%    \end{macrocode}
-% \end{variable}
-%
-% \begin{variable}{\l_@@_numerator_bool}
-%   Indicates that the current unit should go into the numerator when splitting
-%   into two parts (fractions or other \enquote{sorted} styles).
-%    \begin{macrocode}
-\bool_new:N \l_@@_numerator_bool
-%    \end{macrocode}
-% \end{variable}
-%
-% \begin{variable}{\l_@@_qualifier_mode_tl}
-%   For storing the text of options which are best handled by picking
-%   function names.
-%    \begin{macrocode}
-\tl_new:N \l_@@_qualifier_mode_tl
-%    \end{macrocode}
-% \end{variable}
-%
-% \begin{variable}{\l_@@_prefix_power_bool}
-%   Used to determine if prefixes are converted into powers. Note that
-%   while this may be set as an option \enquote{higher up}, at this point it
-%   is handled as an internal switch (see the two formatting interfaces for
-%   reasons).
-%    \begin{macrocode}
-\bool_new:N \l_@@_prefix_power_bool
-%    \end{macrocode}
-% \end{variable}
-%
-% \begin{variable}{\l_@@_prefix_fp}
-%   When converting prefixes to powers, the calculations are done as an
-%   \texttt{fp}.
-%    \begin{macrocode}
-\fp_new:N \l_@@_prefix_fp
-%    \end{macrocode}
-% \end{variable}
-%
-% \begin{variable}{\l_@@_current_tl, \l_@@_part_tl}
-%   Building up the (partial) formatted unit requires some token list storage.
-%   Each part of the unit combination that is recovered also has to be
-%   placed in a token list: this is a dedicated one to leave the scratch
-%   variables available.
-%    \begin{macrocode}
-\tl_new:N \l_@@_current_tl
-\tl_new:N \l_@@_part_tl
-%    \end{macrocode}
-% \end{variable}
-%
-% \begin{variable}{\l_@@_denominator_tl}
-%   For fraction-like units, space is needed for the denominator as well as
-%   the numerator (which is handled using \cs{l_@@_formatted_tl}).
-%    \begin{macrocode}
-\tl_new:N \l_@@_denominator_tl
-%    \end{macrocode}
-% \end{variable}
-%
-% \begin{variable}{\l_@@_total_int}
-%   The formatting routine needs to know both the total number of units and
-%   the current unit. Thus an \texttt{int} is required in addition to
-%   \cs{l_@@_position_int}.
-%    \begin{macrocode}
-\int_new:N \l_@@_total_int
-%    \end{macrocode}
-% \end{variable}
-%
-% \begin{macro}{\@@_format_parsed:}
-% \begin{macro}{\@@_format_parsed_aux:n}
-%   The main formatting routine is essentially a loop over each position,
-%   reading the various parts of the unit to build up complete unit
-%   combination.
-%    \begin{macrocode}
-\cs_new_protected:Npn \@@_format_parsed:
-  {
-    \int_set_eq:NN \l_@@_total_int \l_@@_position_int
-    \tl_clear:N \l_@@_denominator_tl
-    \tl_clear:N \l_@@_formatted_tl
-    \fp_zero:N \l_@@_prefix_fp
-    \int_zero:N \l_@@_position_int
-    \int_do_while:nNnn
-      \l_@@_position_int < \l_@@_total_int
-      {
-        \bool_set_false:N \l_@@_bracket_bool
-        \tl_clear:N \l_@@_current_tl
-        \bool_set_false:N \l_@@_font_bool
-        \bool_set_true:N \l_@@_numerator_bool
-        \int_incr:N \l_@@_position_int
-        \clist_map_inline:nn { prefix , unit , qualifier , power , special }
-          { \@@_format_parsed_aux:n {##1} }
-        \@@_format_output:
-      }
-    \@@_format_finalise:
-  }
-\cs_new_protected:Npn \@@_format_parsed_aux:n #1
-  {
-    \tl_set:Nx \l_@@_tmp_tl
-      { #1 - \int_use:N \l_@@_position_int }
-    \prop_get:NVNT \l_@@_parsed_prop
-      \l_@@_tmp_tl \l_@@_part_tl
-      { \use:c { @@_format_ #1 : } }
-  }
-%    \end{macrocode}
-% \end{macro}
-% \end{macro}
-%
-% \begin{macro}[EXP]{\@@_format_bracket:N}
-%   A quick utility function which wraps up a token list variable in brackets
-%   if they are required.
-%    \begin{macrocode}
-\cs_new:Npn \@@_format_bracket:N #1
-  {
-    \bool_if:NTF \l_@@_bracket_bool
-      {
-        \exp_not:V \l_@@_bracket_open_tl
-        \exp_not:V #1
-        \exp_not:V \l_@@_bracket_close_tl
-      }
-      { \exp_not:V #1 }
-  }
-%    \end{macrocode}
-% \end{macro}
-%
-% \begin{macro}{\@@_format_power:}
-% \begin{macro}[EXP]{\@@_format_power_aux:wTF}
-% \begin{macro}
-%   {
-%     \@@_format_power_positive: ,
-%     \@@_format_power_negative:
-%   }
-% \begin{macro}[EXP]{\@@_format_power_negative_aux:w}
-% \begin{macro}{\@@_format_power_superscript:}
-%   Formatting powers requires a test for negative numbers and depending on
-%   output format requests some adjustment to the stored value. This could be
-%   done using an \texttt{fp} function, but that would be slow compared to
-%   a dedicated if lower-level approach based on delimited arguments.
-%    \begin{macrocode}
-\cs_new_protected:Npn \@@_format_power:
-  {
-    \@@_format_font:
-    \exp_after:wN \@@_format_power_aux:wTF
-      \l_@@_part_tl - \q_stop
-      { \@@_format_power_negative: }
-      { \@@_format_power_positive: }
-  }
-\cs_new:Npn \@@_format_power_aux:wTF #1 - #2 \q_stop
-  { \tl_if_empty:nTF {#1} }
-%    \end{macrocode}
-%   In the case of positive powers, there is little to do: add the power
-%   as a subscript (must be required as the parser ensures it's $\neq 1$).
-%    \begin{macrocode}
-\cs_new_protected:Npn \@@_format_power_positive:
-  { \@@_format_power_superscript: }
-%    \end{macrocode}
-%   Dealing with negative powers starts by flipping the switch used to track
-%   where in the final output the current part should get added to. For the
-%   case where the output is fraction-like, strip off the |~| then ensure that
-%   the result is not the trivial power~$1$. Assuming all is well, addition
-%   to the current unit combination goes ahead.
-%    \begin{macrocode}
-\cs_new_protected:Npn \@@_format_power_negative:
-  {
-    \bool_set_false:N \l_@@_numerator_bool
-    \bool_if:NTF \l_@@_powers_positive_bool
-      {
-        \tl_set:Nx \l_@@_part_tl
-          {
-            \exp_after:wN \@@_format_power_negative_aux:w
-              \l_@@_part_tl \q_stop
-          }
-        \str_if_eq_x:nnF { \exp_not:V \l_@@_part_tl } { 1 }
-          { \@@_format_power_superscript: }
-      }
-      { \@@_format_power_superscript: }
-  }
-\cs_new:Npn \@@_format_power_negative_aux:w - #1 \q_stop
-  { \exp_not:n {#1} }
-%    \end{macrocode}
-%   Adding the power as a superscript has the slight complication that there
-%   is the possibility of needing some brackets. The superscript itself uses
-%   \cs{sp} as that avoids any category code issues and also allows redirection
-%   at a higher level more readily.
-%    \begin{macrocode}
-\cs_new_protected:Npn \@@_format_power_superscript:
-  {
-    \tl_set:Nx \l_@@_current_tl
-      {
-        \@@_format_bracket:N \l_@@_current_tl
-        ^ { \exp_not:V \l_@@_part_tl }
-      }
-    \bool_set_false:N \l_@@_bracket_bool
-  }
-%    \end{macrocode}
-% \end{macro}
-% \end{macro}
-% \end{macro}
-% \end{macro}
-% \end{macro}
-%
-% \begin{macro}{\@@_format_prefix:}
-% \begin{macro}{\@@_format_prefix_power:, \@@_format_prefix_symbol:}
-%   Formatting for prefixes depends on whether they are to be expressed as
-%   symbols or collected up to be returned as a power of $10$. The latter
-%   case requires a bit of processing, which includes checking that the
-%   conversion is possible and allowing for any power that applies to the
-%   current unit.
-%    \begin{macrocode}
-\cs_new_protected:Npn \@@_format_prefix:
-  {
-    \bool_if:NTF \l_@@_prefix_power_bool
-      { \@@_format_prefix_power: }
-      { \@@_format_prefix_symbol: }
-  }
-\cs_new_protected:Npn \@@_format_prefix_power:
-  {
-    \prop_get:NVNTF \l_@@_prefixes_forward_prop
-      \l_@@_part_tl \l_@@_part_tl
-      {
-        \tl_set:Nx \l_@@_tmp_tl
-          { power- \int_use:N \l_@@_position_int }
-        \prop_get:NVNF \l_@@_parsed_prop
-          \l_@@_tmp_tl \l_@@_tmp_tl
-          { \tl_set:Nn \l_@@_tmp_tl { 1 } }
-        \fp_add:Nn \l_@@_prefix_fp
-          { \l_@@_tmp_tl * \l_@@_part_tl }
-      }
-      { \@@_format_prefix_symbol: }
-  }
-\cs_new_protected:Npn \@@_format_prefix_symbol:
-  { \tl_set_eq:NN \l_@@_current_tl \l_@@_part_tl }
-%    \end{macrocode}
-% \end{macro}
-% \end{macro}
-%
-% \begin{macro}{\@@_format_qualifier:}
-% \begin{macro}
-%   {
-%     \@@_format_qualifier_bracket :  ,
-%     \@@_format_qualifier_combine:   ,
-%     \@@_format_qualifier_phrase:    ,
-%     \@@_format_qualifier_subscript:
-%   }
-%  There are various ways that a qualifier can be added to the output. The
-%  idea here is to modify the \enquote{base} text appropriately and then add
-%  to the current unit. In the case that a linking phrase is in use, the
-%  addition of spaces means that the unit may end up ambiguous, and brackets
-%  are therefore required \emph{if} there is a power. Notice that when the
-%  qualifier is just treated as \enquote{text}, the auxiliary is actually
-%  a no-op.
-%    \begin{macrocode}
-\cs_new_protected:Npn \@@_format_qualifier:
-  {
-    \use:c
-      {
-        @@_format_qualifier_
-        \l_@@_qualifier_mode_tl :
-      }
-    \tl_put_right:NV \l_@@_current_tl \l_@@_part_tl
-  }
-\cs_new_protected:Npn \@@_format_qualifier_bracket:
-  {
-    \@@_format_font:
-    \tl_set:Nx \l_@@_part_tl
-      {
-        \exp_not:V \l_@@_bracket_open_tl
-        \exp_not:N \mathrm
-          { \exp_not:V \l_@@_part_tl }
-        \exp_not:V \l_@@_bracket_close_tl
-      }
-  }
-\cs_new_protected:Npn \@@_format_qualifier_combine: { }
-\cs_new_protected:Npn \@@_format_qualifier_phrase:
-  {
-    \@@_format_font:
-    \tl_set:Nx \l_@@_part_tl
-      {
-        \exp_not:V \l_@@_qualifier_phrase_tl
-        \exp_not:N \mathrm
-          { \exp_not:V \l_@@_part_tl }
-      }
-  }
-\cs_new_protected:Npn \@@_format_qualifier_subscript:
-  {
-    \@@_format_font:
-    \tl_set:Nx \l_@@_part_tl
-      {
-        \c_@@_math_subscript_tl
-          {
-            \exp_not:N \mathrm
-              { \exp_not:V \l_@@_part_tl }
-          }
-      }
-  }
-%    \end{macrocode}
-% \end{macro}
-% \end{macro}
-%
-% \begin{macro}{\@@_format_special:}
-%   Any special odds and ends are handled by simply making the current
-%   combination into an argument for the recovered code.
-%    \begin{macrocode}
-\cs_new_protected:Npn \@@_format_special:
-  {
-    \tl_set:Nx \l_@@_current_tl
-      {
-        \exp_not:V \l_@@_part_tl
-          { \exp_not:V \l_@@_current_tl }
-      }
-  }
-%    \end{macrocode}
-% \end{macro}
-%
-% \begin{macro}{\@@_format_unit:}
-%   A very simple task: add the unit to the output currently being
-%   constructed.
-%    \begin{macrocode}
-\cs_new_protected:Npn \@@_format_unit:
-  {
-    \tl_put_right:NV
-      \l_@@_current_tl \l_@@_part_tl
-  }
-%    \end{macrocode}
-% \end{macro}
-%
-% \begin{macro}{\@@_format_output:}
-% \begin{macro}
-%   {\@@_format_output_aux:, \@@_format_output_denominator:}
-% \begin{macro}
-%   {
-%     \@@_format_output_aux:nn ,
-%     \@@_format_output_aux:nV ,
-%     \@@_format_output_aux:nv
-%   }
-%   The first step here is to make a choice based on whether the current
-%   part should be stored as part of the numerator or denominator of a
-%   fraction. In all cases, if the switch \cs{l_@@_numerator_bool} is
-%   true then life is simple: add the current part to the numerator with
-%   a standard separator
-%    \begin{macrocode}
-\cs_new_protected:Npn \@@_format_output:
-  {
-    \@@_format_font:
-    \bool_set_false:N \l_@@_bracket_bool
-    \use:c
-      {
-        @@_format_output_
-        \bool_if:NTF \l_@@_numerator_bool
-          { aux: }
-          { denominator: }
-      }
-  }
-\cs_new_protected:Npn \@@_format_output_aux:
-  {
-    \@@_format_output_aux:nV { formatted }
-      \l_@@_product_tl
-  }
-%    \end{macrocode}
-%   There are a few things to worry about at this stage if the current part
-%   is in the denominator.  Powers have already been dealt with and some
-%   formatting outcomes only need a branch at the final point of building
-%   the entire unit. That means that there are three possible outcomes here:
-%   if collecting two separate parts, add to the denominator with a product
-%   separator, or if only building one token list there may be a need to use
-%   a symbol separator. When the |repeated-symbol| option is in use there may
-%   be a need to add a leading |1| to the output in the case where the
-%   first unit is in the denominator: that can be picked up by looking for
-%   empty output in combination with the flag for using a symbol in the output
-%   but not a two-part strategy.
-%    \begin{macrocode}
-\cs_new_protected:Npn \@@_format_output_denominator:
-  {
-    \bool_if:NTF \l_@@_two_part_bool
-      {
-        \bool_lazy_and:nnT
-          { \l_@@_denominator_bracket_bool }
-          { ! \tl_if_empty_p:N \l_@@_denominator_tl }
-          { \bool_set_true:N \l_@@_bracket_bool }
-        \@@_format_output_aux:nV { denominator }
-          \l_@@_product_tl
-      }
-      {
-        \bool_lazy_and:nnT
-          { \l_@@_per_symbol_bool }
-          { \tl_if_empty_p:N \l_@@_formatted_tl }
-          { \tl_set:Nn \l_@@_formatted_tl { 1 } }
-        \@@_format_output_aux:nv { formatted }
-          {
-            l_@@_
-            \bool_if:NTF \l_@@_per_symbol_bool
-              { per_symbol }
-              { product }
-            _tl
-          }
-      }
-  }
-\cs_new_protected:Npn \@@_format_output_aux:nn #1#2
-  {
-    \tl_set:cx { l_@@_ #1 _tl }
-      {
-         \exp_not:v { l_@@_ #1 _tl }
-         \tl_if_empty:cF { l_@@_ #1 _tl }
-           { \exp_not:n {#2} }
-         \exp_not:V \l_@@_current_tl
-      }
-  }
-\cs_generate_variant:Nn \@@_format_output_aux:nn { nV , nv }
-%    \end{macrocode}
-% \end{macro}
-% \end{macro}
-% \end{macro}
-%
-% \begin{macro}{\@@_format_font:}
-%   A short auxiliary which checks if the font has been applied to the
-%   main part of the output: if not, add it and set the flag.
-%    \begin{macrocode}
-\cs_new_protected:Npn \@@_format_font:
-  {
-    \bool_if:NF \l_@@_font_bool
-      {
-        \tl_set:Nx \l_@@_current_tl
-          {
-            \exp_not:N \mathrm
-              { \exp_not:V \l_@@_current_tl }
-          }
-        \bool_set_true:N \l_@@_font_bool
-      }
-  }
-%    \end{macrocode}
-% \end{macro}
-%
-% \begin{macro}{\@@_format_finalise:}
-% \begin{macro}
-%   {
-%     \@@_format_finalise_autofrac:   ,
-%     \@@_format_finalise_fractional: ,
-%     \@@_format_finalise_power:
-%   }
-%   Finalising the unit format is really about picking up the cases involving
-%   fractions: these require assembly of the parts with the need to add
-%   additional material in some cases
-%    \begin{macrocode}
-\cs_new_protected:Npn \@@_format_finalise:
-  {
-    \tl_if_empty:NF \l_@@_denominator_tl
-      {
-        \bool_if:NTF \l_@@_powers_positive_bool
-          { \@@_format_finalise_fractional: }
-          { \@@_format_finalise_power: }
-      }
-  }
-%    \end{macrocode}
-%   For fraction-like output, there are three possible choices and two
-%   actual styles. In all cases, if the numerator is empty then it is set
-%   here to |1|.  To deal with the \enquote{auto-format} case, the two
-%   styles (fraction and symbol) are handled in auxiliaries: this allows both
-%   to be used at the same time! Beyond that, the key here is to use a
-%   single \cs{tl_set:Nx} to keep down the number of assignments.
-%    \begin{macrocode}
-\cs_new_protected:Npn \@@_format_finalise_fractional:
-  {
-    \tl_if_empty:NT \l_@@_formatted_tl
-      { \tl_set:Nn \l_@@_formatted_tl { 1 } }
-    \bool_if:NTF \l_@@_autofrac_bool
-      { \@@_format_finalise_autofrac: }
-      {
-        \bool_if:NTF \l_@@_per_symbol_bool
-          { \@@_format_finalise_symbol: }
-          { \@@_format_finalise_fraction: }
-      }
-  }
-%    \end{macrocode}
-%   For the \enquote{auto-selected} fraction method, the two other auxiliary
-%   functions are used to do both forms of formatting. So that everything
-%   required is available, this needs one group so that the second auxiliary
-%   receives the correct input. After that it is just a case of applying
-%   \cs{mathchoice} to the formatted output.
-%    \begin{macrocode}
-\cs_new_protected:Npn \@@_format_finalise_autofrac:
-  {
-    \group_begin:
-      \@@_format_finalise_fraction:
-    \exp_args:NNNV \group_end:
-    \tl_set:Nn \l_@@_tmp_tl \l_@@_formatted_tl
-    \@@_format_finalise_symbol:
-    \tl_set:Nx \l_@@_formatted_tl
-      {
-        \mathchoice
-          { \exp_not:V \l_@@_tmp_tl }
-          { \exp_not:V \l_@@_formatted_tl }
-          { \exp_not:V \l_@@_formatted_tl }
-          { \exp_not:V \l_@@_formatted_tl }
-      }
-  }
-%    \end{macrocode}
-%   When using a fraction function the two parts are now assembled.
-%    \begin{macrocode}
-\cs_new_protected:Npn \@@_format_finalise_fraction:
-  {
-    \tl_set:Nx \l_@@_formatted_tl
-      {
-        \exp_not:V \l_@@_fraction_function_tl
-          { \exp_not:V \l_@@_formatted_tl }
-          { \exp_not:V \l_@@_denominator_tl }
-      }
-  }
-\cs_new_protected:Npn \@@_format_finalise_symbol:
-  {
-    \tl_set:Nx \l_@@_formatted_tl
-      {
-        \exp_not:V \l_@@_formatted_tl
-        \exp_not:V \l_@@_per_symbol_tl
-        \@@_format_bracket:N \l_@@_denominator_tl
-      }
-  }
-%    \end{macrocode}
-%   In the case of sorted powers, there is a test to make sure there was
-%   at least one positive power, and if so a simple join of the two parts
-%   with the appropriate product.
-%    \begin{macrocode}
-\cs_new_protected:Npn \@@_format_finalise_power:
-  {
-    \tl_if_empty:NTF \l_@@_formatted_tl
-      {
-        \tl_set_eq:NN
-          \l_@@_formatted_tl
-          \l_@@_denominator_tl
-      }
-      {
-        \tl_set:Nx \l_@@_formatted_tl
-          {
-            \exp_not:V \l_@@_formatted_tl
-            \exp_not:V \l_@@_product_tl
-            \exp_not:V \l_@@_denominator_tl
-          }
-      }
-  }
-%    \end{macrocode}
-% \end{macro}
-% \end{macro}
-%
-% \subsection{Pre-defined unit components}
-%
-% Quite a number of units can be predefined: while this is a code-level module,
-% there is little point having a unit parser which does not start off able to
-% parse any units!
-%
-% \begin{macro}
-%   {
-%     \kilogram ,
-%     \metre    ,
-%     \meter    ,
-%     \mole     ,
-%     \kelvin   ,
-%     \candela  ,
-%     \second   ,
-%     \ampere
-%   }
-%   The basic SI units: technically the correct spelling is \cs{metre} but
-%   US users tend to use \cs{meter}.
-%    \begin{macrocode}
-\siunitx_declare_unit:Nn \kilogram { \kilo \gram }
-\siunitx_declare_unit:Nn \metre    { m }
-\siunitx_declare_unit:Nn \meter    { \metre }
-\siunitx_declare_unit:Nn \mole     { mol }
-\siunitx_declare_unit:Nn \second   { s }
-\siunitx_declare_unit:Nn \ampere   { A }
-\siunitx_declare_unit:Nn \kelvin   { K }
-\siunitx_declare_unit:Nn \candela  { cd }
-%    \end{macrocode}
-% \end{macro}
-%
-% \begin{macro}{\gram}
-%   The gram is an odd unit as it is needed for the base unit kilogram.
-%    \begin{macrocode}
-\siunitx_declare_unit:Nn \gram { g }
-%    \end{macrocode}
-% \end{macro}
-%
-% \begin{macro}
-%   {
-%     \yocto ,
-%     \zepto ,
-%     \atto  ,
-%     \femto ,
-%     \pico  ,
-%     \nano  ,
-%     \micro ,
-%     \milli ,
-%     \centi ,
-%     \deci
-%  }
-%   The various SI multiple prefixes are defined here: first the small
-%   ones.
-%    \begin{macrocode}
-\siunitx_declare_prefix:Nnn \yocto { y } { -24 }
-\siunitx_declare_prefix:Nnn \zepto { z } { -21 }
-\siunitx_declare_prefix:Nnn \atto  { a } { -18 }
-\siunitx_declare_prefix:Nnn \femto { f } { -15 }
-\siunitx_declare_prefix:Nnn \pico  { p } { -12 }
-\siunitx_declare_prefix:Nnn \nano  { n } { -9 }
-\siunitx_declare_prefix:Nnn \micro { [micro] } { -6 }
-\siunitx_declare_prefix:Nnn \milli { m } { -3 }
-\siunitx_declare_prefix:Nnn \centi { c } { -2 }
-\siunitx_declare_prefix:Nnn \deci  { d } { -1 }
-%    \end{macrocode}
-% \end{macro}
-% \begin{macro}
-%   {
-%     \deca  ,
-%     \deka  ,
-%     \hecto ,
-%     \kilo  ,
-%     \mega  ,
-%     \giga  ,
-%     \tera  ,
-%     \peta  ,
-%     \exa   ,
-%     \zetta ,
-%     \yotta
-%   }
-%   Now the large ones.
-%    \begin{macrocode}
-\siunitx_declare_prefix:Nnn \deca  { da } { 1 }
-\siunitx_declare_prefix:Nnn \deka  { da } { 1 }
-\siunitx_declare_prefix:Nnn \hecto { h }  { 2 }
-\siunitx_declare_prefix:Nnn \kilo  { k }  { 3 }
-\siunitx_declare_prefix:Nnn \mega  { M }  { 6 }
-\siunitx_declare_prefix:Nnn \giga  { G }  { 9 }
-\siunitx_declare_prefix:Nnn \tera  { T }  { 12 }
-\siunitx_declare_prefix:Nnn \peta  { P }  { 15 }
-\siunitx_declare_prefix:Nnn \exa   { E }  { 18 }
-\siunitx_declare_prefix:Nnn \zetta { Z }  { 21 }
-\siunitx_declare_prefix:Nnn \yotta { Y }  { 24 }
-%    \end{macrocode}
-% \end{macro}
-%
-% \begin{macro}
-%   {
-%     \becquerel     ,
-%     \degreeCelsius ,
-%     \coulomb       ,
-%     \farad         ,
-%     \gray          ,
-%     \hertz         ,
-%     \henry         ,
-%     \joule         ,
-%     \katal         ,
-%     \lumen         ,
-%     \lux
-%  }
-%    Named derived units: first half of alphabet.
-%    \begin{macrocode}
-\siunitx_declare_unit:Nn \becquerel { Bq }
-\siunitx_declare_unit:Nn \degreeCelsius
-  { \ensuremath { { } ^ { \circ } } \kern -\scriptspace C }
-\siunitx_declare_unit:Nn \coulomb   { C }
-\siunitx_declare_unit:Nn \farad     { F }
-\siunitx_declare_unit:Nn \gray      { Gy }
-\siunitx_declare_unit:Nn \hertz     { Hz }
-\siunitx_declare_unit:Nn \henry     { H }
-\siunitx_declare_unit:Nn \joule     { J }
-\siunitx_declare_unit:Nn \katal     { kat }
-\siunitx_declare_unit:Nn \lumen     { lm }
-\siunitx_declare_unit:Nn \lux       { lx }
-%    \end{macrocode}
-% \end{macro}
-% \begin{macro}
-%   {
-%     \newton    ,
-%     \ohm       ,
-%     \pascal    ,
-%     \radian    ,
-%     \siemens   ,
-%     \sievert   ,
-%     \steradian ,
-%     \tesla     ,
-%     \volt      ,
-%     \watt      ,
-%     \weber
-%  }
-%    Named derived units: second half of alphabet.
-%    \begin{macrocode}
-\siunitx_declare_unit:Nn \newton    { N }
-\siunitx_declare_unit:Nn \ohm       { \ensuremath { \Omega } }
-\siunitx_declare_unit:Nn \pascal    { Pa }
-\siunitx_declare_unit:Nn \radian    { rad }
-\siunitx_declare_unit:Nn \siemens   { S }
-\siunitx_declare_unit:Nn \sievert   { Sv }
-\siunitx_declare_unit:Nn \steradian { sr }
-\siunitx_declare_unit:Nn \tesla     { T }
-\siunitx_declare_unit:Nn \volt      { V }
-\siunitx_declare_unit:Nn \watt      { W }
-\siunitx_declare_unit:Nn \weber     { Wb }
-%    \end{macrocode}
-% \end{macro}
-%
-% \begin{macro}
-%   {
-%     \day       ,
-%     \hectare   ,
-%     \hour      ,
-%     \litre     ,
-%     \liter     ,
-%     \minute    ,
-%     \tonne
-%  }
-%   Non-SI, but accepted for general use. Once again there are two
-%   spellings, here for litre and with different output in this case.
-%    \begin{macrocode}
-\siunitx_declare_unit:Nn \day     { d }
-\siunitx_declare_unit:Nn \hectare { ha }
-\siunitx_declare_unit:Nn \hour    { h }
-\siunitx_declare_unit:Nn \litre   { L }
-\siunitx_declare_unit:Nn \liter   { \litre }
-\siunitx_declare_unit:Nn \minute  { min }
-\siunitx_declare_unit:Nn \tonne   { t }
-%    \end{macrocode}
-% \end{macro}
-% \begin{macro}
-%   {
-%     \arcminute ,
-%     \arcsecond ,
-%     \degree
-%  }
-%   Arc units: again, non-SI, but accepted for general use.
-%    \begin{macrocode}
-\siunitx_declare_unit:Nn \arcminute { \ensuremath { ^ { \prime } } }
-\siunitx_declare_unit:Nn \arcsecond { \ensuremath { ^ { \prime \prime } } }
-\siunitx_declare_unit:Nn \degree    { \ensuremath { ^ { \circ } } }
-%    \end{macrocode}
-% \end{macro}
-%
-% \begin{macro}{\astronomicalunit, \atomicmassunit, \dalton, \electronvolt}
-%   A few units based on physical measurements exist: these ones are accepted
-%   for use with the International System.
-%    \begin{macrocode}
-\siunitx_declare_unit:Nn \astronomicalunit { au }
-\siunitx_declare_unit:Nn \atomicmassunit   { u }
-\siunitx_declare_unit:Nn \dalton           { Da }
-\siunitx_declare_unit:Nn \electronvolt     { eV }
-%    \end{macrocode}
-% \end{macro}
-% \begin{macro}{\nuaction, \numass, \nuspeed, \nutime}
-%   Natural units based on physical constants.
-%    \begin{macrocode}
-\siunitx_declare_unit:Nn \nuaction { \ensuremath { \mathit { \hbar } } }
-\siunitx_declare_unit:Nx \numass
-  {
-    \exp_not:N \ensuremath
-      {
-        \exp_not:N \mathit { m }
-          \c_@@_math_subscript_tl { \exp_not:N \mathrm { e } }
-      }
-  }
-\siunitx_declare_unit:Nx \nuspeed
-  {
-    \exp_not:N \ensuremath
-      { \exp_not:N \mathit { c } \c_@@_math_subscript_tl { 0 } }
-  }
-\siunitx_declare_unit:Nn \nutime
-  { \numass \per \numass \per \nuspeed \squared }
-%    \end{macrocode}
-% \end{macro}
-% \begin{macro}
-%   {
-%     \auaction ,
-%     \aucharge ,
-%     \auenergy ,
-%     \aulength ,
-%     \aumass   ,
-%     \autime   ,
-%     \bohr     ,
-%     \hartree
-%   }
-%   Atomic units based on physical constants.
-%    \begin{macrocode}
-\siunitx_declare_unit:Nn \auaction { \ensuremath { \mathit { \hbar } } }
-\siunitx_declare_unit:Nn \aucharge { \ensuremath { \mathit { e } } }
-\siunitx_declare_unit:Nx \auenergy
-  {
-    \exp_not:N \ensuremath
-      {
-        \exp_not:N \mathit { E }
-          \c_@@_math_subscript_tl { \exp_not:N \mathrm { h } }
-      }
-  }
-\siunitx_declare_unit:Nx \aulength
-  {
-    \exp_not:N  \ensuremath
-      { \exp_not:N \mathit { a } \c_@@_math_subscript_tl { 0 } }
-  }
-\siunitx_declare_unit:Nx \aumass
-  {
-    \exp_not:N \ensuremath
-      {
-        \exp_not:N \mathit { m }
-          \c_@@_math_subscript_tl { \exp_not:N \mathrm { e } }
-      }
-  }
-\siunitx_declare_unit:Nn \autime   { \auaction \per \auenergy }
-\siunitx_declare_unit:Nn \bohr     { \aulength }
-\siunitx_declare_unit:Nn \hartree  { \auenergy }
-%    \end{macrocode}
-% \end{macro}
-%
-% \begin{macro}
-%   {
-%     \angstrom          ,
-%     \bar               ,
-%     \barn              ,
-%     \bel               ,
-%     \decibel           ,
-%     \knot              ,
-%     \millimetremercury ,
-%     \nauticalmile      ,
-%     \neper
-%  }
-%   There are then some day-to-day units which are accepted for use
-%   with SI, but are not part of the official specification.
-%    \begin{macrocode}
-\siunitx_declare_unit:Nn \angstrom          { \mbox { \AA } }
-\siunitx_declare_unit:Nn \bar               { bar }
-\siunitx_declare_unit:Nn \barn              { b }
-\siunitx_declare_unit:Nn \bel               { B }
-\siunitx_declare_unit:Nn \decibel           { \deci \bel }
-\siunitx_declare_unit:Nn \knot              { kn }
-\siunitx_declare_unit:Nn \millimetremercury { mmHg }
-\siunitx_declare_unit:Nn \nauticalmile      { M }
-\siunitx_declare_unit:Nn \neper             { Np }
-%    \end{macrocode}
-% \end{macro}
-%
-% \begin{macro}
-%   {
-%     \dyne    ,
-%     \erg     ,
-%     \gal     ,
-%     \gauss   ,
-%     \maxwell ,
-%     \oersted ,
-%     \phot    ,
-%     \poise   ,
-%     \stilb   ,
-%     \stokes
-%  }
-%   \textsc{cgs} units: similar to the set immediately above, these may be used
-%   for specific applications.
-%    \begin{macrocode}
-\siunitx_declare_unit:Nn \dyne    { dyn  }
-\siunitx_declare_unit:Nn \erg     { erg }
-\siunitx_declare_unit:Nn \gal     { Gal }
-\siunitx_declare_unit:Nn \gauss   { G }
-\siunitx_declare_unit:Nn \maxwell { Mx }
-\siunitx_declare_unit:Nn \oersted { Oe }
-\siunitx_declare_unit:Nn \phot    { ph }
-\siunitx_declare_unit:Nn \poise   { P }
-\siunitx_declare_unit:Nn \stilb   { sb }
-\siunitx_declare_unit:Nn \stokes  { St }
-%    \end{macrocode}
-% \end{macro}
-%
-% \begin{macro}{\percent}
-%   For percent, the raw character is the most flexible way of handling output.
-%    \begin{macrocode}
-\siunitx_declare_unit:Nn \percent  { \char "25 ~ }
-%    \end{macrocode}
-% \end{macro}
-%
-% \begin{macro}{\square, \squared, \cubic, \cubed}
-%   Basic powers.
-%    \begin{macrocode}
-\siunitx_declare_power:NNn \square \squared { 2 }
-\siunitx_declare_power:NNn \cubic  \cubed   { 3 }
-%    \end{macrocode}
-% \end{macro}
-%
-% \subsection{Messages}
-%
-%    \begin{macrocode}
-\msg_new:nnnn { siunitx } { unit / dangling-part }
-  { Found~#1~part~with~no~unit. }
-  {
-    Each~#1~part~must~be~associated~with~a~unit:~a~#1~part~was~found~
-    but~no~following~unit~was~given.
-  }
-\msg_new:nnnn { siunitx } { unit / duplicate-part }
-  { Duplicate~#1~part:~#2. }
-  {
-    Each~unit~may~have~only~one~#1:\\
-    the~additional~#1~part~'#2'~will~be~ignored.
-  }
-\msg_new:nnnn { siunitx } { unit / duplicate-sticky-per }
-  { Duplicate~\token_to_str:N \per. }
-  {
-    When~the~'sticky-per'~option~is~active,~only~one~
-    \token_to_str:N \per \  may~appear~in~a~unit.
-  }
-\msg_new:nnnn { siunitx } { unit / part-before-unit }
-  { Found~#1~part~before~first~unit:~#2. }
-  {
-    The~#1~part~'#2'~must~follow~after~a~unit:~
-    it~cannot~appear~before~any~units~and~will~therefore~be~ignored.
-  }
-%    \end{macrocode}
-%
-% \subsection{Standard settings for module options}
-%
-% Some of these follow naturally from the point of definition
-% (\emph{e.g.}~boolean variables are always |false| to begin with),
-% but for clarity everything is set here.
-%    \begin{macrocode}
-\keys_set:nn { siunitx }
-  {
-    bracket-denominator = true      ,
-    fraction-command    = \frac     ,
-    parse-units         = true      ,
-    per-mode            = power     ,
-    per-symbol          = /         ,
-    qualifier-mode      = subscript ,
-    qualifier-phrase    = \         ,
-    sticky-per          = false     ,
-    unit-close-bracket  = )         , % (
-    unit-open-bracket   = (         , % )
-    unit-product        = \,
-  }
-%    \end{macrocode}
-%
-%    \begin{macrocode}
-%</package>
-%    \end{macrocode}
-%
-% \end{implementation}
-%
-% \begin{thebibliography}{1}
-%   \bibitem{BIPM}
-%     \emph{The International System of Units (SI)},
-%     \url{https://www.bipm.org/en/measurement-units/}.
-%   \bibitem{SI:2.1}
-%     \emph{SI base units},
-%     \url{https://www.bipm.org/en/publications/si-brochure/section2-1.html}.
-%   \bibitem{SI:2.2.2}
-%     \emph{Units with special names and symbols; units that
-%       incorporate special names and symbols},
-%     \url{https://www.bipm.org/en/publications/si-brochure/section2-2-2.html}.
-%   \bibitem{SI:3.1}
-%     \emph{SI Prefixes},
-%     \url{https://www.bipm.org/en/publications/si-brochure/chapter3.html}.
-%   \bibitem{SI:5.3.7}
-%     \emph{Stating values of dimensionless quantities, or quantities of
-%       dimension one},
-%     \url{https://www.bipm.org/en/publications/si-brochure/section5-3-7.html}.
-%   \bibitem{SI:T6}
-%     \emph{Non-SI units accepted for use with the International
-%       System of Units},
-%     \url{https://www.bipm.org/en/publications/si-brochure/table6.html}.
-%   \bibitem{SI:T7}
-%     \emph{Non-SI units whose values in SI units must be obtained
-%       experimentally},
-%     \url{https://www.bipm.org/en/publications/si-brochure/table7.html}.
-%   \bibitem{SI:T8}
-%     \emph{Other non-SI units},
-%     \url{https://www.bipm.org/en/publications/si-brochure/table8.html}.
-%   \bibitem{SI:T9}
-%     \emph{Non-SI units associated with the CGS and the CGS-Gaussian
-%       system of units},
-%     \url{https://www.bipm.org/en/publications/si-brochure/table9.html}.
-% \end{thebibliography}
-%
-% \PrintIndex