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+%% skmath improved math commands
+%%
+%% Copyright (C) 2012-2013 by Simon Sigurdhsson <sigurdhsson@gmail.com>
+%% 
+%% 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 Simon Sigurdhsson.
+%% 
+%% This work consists of the file skmath.tex
+%% and the derived file skmath.sty.
+\PassOptionsToPackage{intlimits,sumlimits,namelimits}{amsmath}
+\documentclass[commonsets,load]{skdoc}
+\usepackage{amsmath}
+\usepackage{csquotes}
+\ProvideDocumentCommand\d{m}{#1} 
+\ProvideDocumentCommand\pd{smm}{#2#3} 
+\ProvideDocumentCommand\abs{m}{#1} 
+\ProvideDocumentCommand\norm{m}{#1} 
+\ProvideDocumentCommand\abs{m}{#1} 
+\ProvideDocumentCommand\P{m}{#1}
+\ProvideDocumentCommand\given{}{} 
+\ProvideDocumentCommand\E{m}{#1} 
+\ProvideDocumentCommand\var{m}{#1} 
+\ProvideDocumentCommand\cov{mm}{#1#2} 
+\ProvideDocumentCommand\N{}{} 
+\ProvideDocumentCommand\Z{}{} 
+\ProvideDocumentCommand\Q{}{} 
+\ProvideDocumentCommand\R{}{} 
+\ProvideDocumentCommand\C{}{} 
+\ProvideDocumentCommand\sfrac{mm}{#1#2} 
+\ProvideDocumentCommand\argmin{om}{#1} 
+\ProvideDocumentCommand\argmax{om}{#1} 
+
+% Declare the target files
+\SelfPreambleTo{\mypreamble}
+\DeclareFile[key=package,preamble=\mypreamble]{skmath.sty}
+
+% Hack for LaTeX3 code
+\ExplSyntaxOn
+\cs_set_protected_nopar:Npn\ExplHack{
+\char_set_catcode_letter:n{ 58 }
+\char_set_catcode_letter:n{ 95 } }
+\ExplSyntaxOff
+
+% This is where the documentation begins
+\begin{document}
+  % Change & version info
+  \version{0.3}
+  \changes{0.1}{Initial version}
+  \changes{0.1c}{Moved package from \pkg{docstrip} to \pkg{skdoc}}
+  \changes{0.1d}{Fixed fatal documentation and package errors}
+  \changes{0.1e}{Added statistics commands}
+  \changes{0.1g}{Documentation fixes}
+  \changes{0.2}{Use \pkg{expl3} functionality throughout the package}
+  \changes{0.3}{Added \cs{min}/\cs{max} and friends. Added \cs{pd}}
+
+  % Metadata
+  \package[ctan=skmath,vcs=https://github.com/urdh/skmath]{skmath}
+  \author{Simon Sigurdhsson}
+  \email{sigurdhsson@gmail.com}
+
+  % First page
+  \maketitle
+  \begin{abstract}
+    The \thepackage\ package provides improved and new math commands
+    for superior typesetting with less effort.
+  \end{abstract}
+
+  \section{Introduction}
+  This package intends to provide helpful (re-)definitions of commands
+  related to typesetting mathematics, and specifically typesetting
+  them in a more intuitive, less verbose and more beautiful way.
+  It was originally not intended for use by the public, and as such
+  there may be incompatibilities with other packages of which I am
+  not aware, but I figured it could be useful to other people as well.
+
+  \section{Usage}
+  \subsection{Options}
+  As of version \theversion, there is only one option: \opt{commonsets}.
+  By default, it is disabled but if the option is given the package will
+  define \cs{N}, \cs{Z}, \cs{Q}, \cs{R} and \cs{C} as blackboard 
+  variants of the respective letters, to represent the common sets
+  of numbers.
+
+  \subsection{New commands}
+  The package defines a number of new commands that aid in typesetting
+  certain mathematical formulae.
+ 
+  \DescribeMacro\N 
+  \DescribeMacro\Z 
+  \DescribeMacro\Q 
+  \DescribeMacro\R 
+  \DescribeMacro\C
+  These commands are only available if the \opt{commonsets}
+  option is given. They typeset the set of natural, integer, rational,
+  real and complex numbers respectively.
+\begin{example}
+\begin{equation*}
+  \N, \Z, \Q, \R, \C.
+\end{equation*}
+\end{example}
+ 
+  \DescribeMacro\norm{<expression>}
+  \DescribeMacro\abs{<expression>}
+  The commands \Macro\norm and \Macro\abs, quite expectedly, typeset
+  the norm ans absolute value of an expression, respectively. They
+  have one mandatory argument (the expression), and different norms
+  can be achieved by appending a subscript after the argument of 
+  \Macro\norm.
+\begin{example}
+\begin{equation*}
+  \norm{\vec{x}}_p =
+    \left(\sum_{i=1}^n \abs{x_i}^p\right)%
+      ^{\sfrac{1}{p}}
+\end{equation*}
+\end{example}
+ 
+  \DescribeMacro\d{<variable>}
+  There is also a command \Macro\d, with one mandatory argument, that 
+  typesets the differential part of an integral.
+\begin{example}
+\begin{equation*}
+  \int_{\R}\! \frac{\sin{x}}{x} \d{x}
+\end{equation*}
+\end{example}
+
+  \DescribeMacro\pd*{<function>}{<var>,\meta{var},...}
+  This macro typesets a partial derivative. The starred variant typesets
+  derivatives as subscripts, i.e. \(\pd*{f}{x^2,y}\), while the unstarred
+  variant typesets full fractions:
+\begin{example}
+\begin{equation*}
+  \pd{f}{x^m,y^n}
+\end{equation*}
+\end{example}
+
+  As the example shows, the comma-separated list of variables also
+  supports superscripts to denote the number of derivatives, and the
+  sum of the variables is automatically calculated.
+
+  \DescribeMacro\E{<expression>}
+  The command \Macro\E typesets the expectation of a random variable.
+\begin{example}
+\begin{equation*}
+  \E{\hat{\mu}} = \mu
+\end{equation*}
+\end{example}
+
+  \DescribeMacro\P{<expression>\AlsoMacro\given <expression>}
+  The \Macro\P command typesets a probability. The \Macro\given command 
+  can be used to typeset conditional probabilities, within \Macro\P.
+\begin{example}
+\begin{equation*}
+  \P{A\given B} =
+    \frac{\P{B\given A}\P{A}}{\P{B}}
+\end{equation*}
+\end{example} 
+
+  \DescribeMacro\var{<expression>}
+  \DescribeMacro\cov{<expression>}{<expression>}
+  The commands \Macro\var and \Macro\cov typeset the variance and
+  covariance of an expression.
+\begin{example}
+\begin{gather*}
+  \var{X} = \E{(X-\mu)^2}\\
+  \cov{X}{Y} = \E{XY}-\E{X}\E{Y}
+\end{gather*}
+\end{example}
+ 
+  \subsection{Improved commands}
+  In addition to adding new commands, this package also redefines
+  already existing commands in a mostly backwards-compatible way
+  to improve their usefulness.
+ 
+  \DescribeMacro\sin[<power>]{<expression>} 
+  \DescribeMacro\arcsin{<expression>} 
+  \DescribeMacro\cos[<power>]{<expression>} 
+  \DescribeMacro\arccos{<expression>} 
+  \DescribeMacro\tan[<power>]{<expression>} 
+  \DescribeMacro\arctan{<expression>} 
+  \DescribeMacro\cot[<power>]{<expression>} 
+  The trigonometric functions have been redefined
+  to typeset more easily. They typeset \meta{expression} as an
+  argument of the expression, and (if applicable) \meta{power} as
+  a superscript between the function and its argument,
+  \emph{e.g.} \(\sin[2]{\phi}\).
+ 
+  \DescribeMacro\ln{<expression>}
+  The natural logarithm macro \Macro\ln has also been redefined to 
+  require an argument which is typeset as the argument of the logarithm.
+  \DescribeMacro\log[<base>]{<expression>}
+  The related macro \Macro\log is redefined in a similar way, but also 
+  accepts an optional argument denoting the base of the logarithm:
+  \(\log[2]{x}\).
+ 
+  \DescribeMacro\exp{<expression>}
+  The exponential, \Macro\exp, is redefined to typeset its argument as a
+  superscript of \(e\) in some display styles, and as an argument of
+  \(\mathrm{exp}\) otherwise:
+  \begin{equation*}
+    \exp{\sqrt{2}\exp{x}}
+  \end{equation*}
+
+  \DescribeMacro\min*[<domain>]{<expression>}
+  \DescribeMacro\argmin*[<domain>]{<expression>}
+  \DescribeMacro\max*[<domain>]{<expression>}
+  \DescribeMacro\argmax*[<domain>]{<expression>}
+  \DescribeMacro\sup*[<domain>]{<expression>}
+  \DescribeMacro\inf*[<domain>]{<expression>}
+  The maximum/minimum macros have been redefined in a manner similar to
+  the trigonometric functions. They typeset \meta{expression} inside
+  curly brackets (the starred version omits the brackets), with the
+  optional \meta{domain} typeset in a subscript after the operator
+  (\emph{e.g.} \(\min*[x\in\R_{+}]{f(x)}\)). Argument variants are also
+  provided, and the \meta{expression} is centered underneath the operator
+  if possible:
+  \begin{equation*}
+    \argmin*[x\in\R_{+}]{f(x)}
+  \end{equation*}
+
+  \subsection{Stylistic changes}
+  Some commands have been redefined in a completely backwards-compatible
+  way to improve the end result of their typesetting.
+ 
+  \DescribeMacro\frac{<numerator>}{<denominator>}
+  The \Macro\frac command has been changed to improve typesetting,
+  allowing displaystyle math in some settings.
+ 
+  \DescribeMacro\bar{<expression>}
+  \DescribeMacro\vec{<expression>}
+  The \Macro\bar command has been changed to cover the entire 
+  \meta{expression} (\emph{i.e.} \(\bar{uv}\)), and \Macro\vec has
+  been changed to match the \cs{vectorsym} command provided by 
+  \pkg{isomath}.
+
+  \section{Known issues}
+  A list of current issues is available in the Github repository of this
+  package\footnote{\url{https://github.com/urdh/skmath/issues}}, but as
+  of the release of \theversion, there are no known issues
+  %\begin{description}
+  %    \item[\#?]  ???
+  %\end{description}
+
+  If you discover any bugs in this package, please report them to the issue
+  tracker in the \thepackage\ Github repository.
+
+  \Implementation\ExplHack
+  \section{Implementation}
+  The package implementation is very simple. First, we do the standard
+  \LaTeXe\ preamble thing, then we require some dependencies.
+  \changes{0.1b}{Load \pkg{amsmath} with \texttt{intlimits} option}
+  \changes{0.2a}{Load \pkg{amsmath} with more \texttt{limits} options}
+\begin{MacroCode}{package}
+\RequirePackage{expl3,l3keys2e,xparse}
+\ProvidesExplPackage{skmath}
+    {2013/05/24}{0.3}{improved math commands}
+\PassOptionsToPackage{intlimits,sumlimits,namelimits}{amsmath}
+\RequirePackage{amssymb,mathtools,xfrac,isomath}
+\end{MacroCode}
+
+  We optionally provide commands to typeset common sets.
+  \begin{macro}{\__skmath_define_common_sets:}
+\begin{MacroCode}{package}
+\cs_new:Nn\__skmath_define_common_sets:{
+\end{MacroCode}
+  \begin{macro}{\N}
+  \changes{0.1b}{Moved to \textsf{xparse} command definition}
+\begin{MacroCode}{package}
+  \NewDocumentCommand\N{}{\ensuremath{\mathbb{N}}}
+\end{MacroCode}
+  \end{macro}
+  \begin{macro}{\Z}
+  \changes{0.1b}{Moved to \textsf{xparse} command definition}
+\begin{MacroCode}{package}
+  \NewDocumentCommand\Z{}{\ensuremath{\mathbb{Z}}}
+\end{MacroCode}
+  \end{macro}
+  \begin{macro}{\Q}
+  \changes{0.1b}{Moved to \textsf{xparse} command definition}
+\begin{MacroCode}{package}
+  \NewDocumentCommand\Q{}{\ensuremath{\mathbb{Q}}}
+\end{MacroCode}
+  \end{macro}
+  \begin{macro}{\R}
+  \changes{0.1b}{Moved to \textsf{xparse} command definition}
+\begin{MacroCode}{package}
+  \NewDocumentCommand\R{}{\ensuremath{\mathbb{R}}}
+\end{MacroCode}
+  \end{macro}
+  \begin{macro}{\C}
+  \changes{0.1b}{Moved to \textsf{xparse} command definition}
+\begin{MacroCode}{package}
+  \NewDocumentCommand\C{}{\ensuremath{\mathbb{C}}}
+\end{MacroCode}
+  \end{macro}
+\begin{MacroCode}{package}
+}
+\end{MacroCode}
+  \end{macro}
+ 
+  This is followed by commands to typeset the norm and absolute value.
+  \begin{macro}{\abs}
+\begin{MacroCode}{package}
+\DeclarePairedDelimiter\abs{\lvert}{\rvert}
+\end{MacroCode}
+  \end{macro}
+  \begin{macro}{\norm}
+\begin{MacroCode}{package}
+\DeclarePairedDelimiter\norm{\lVert}{\rVert}
+\end{MacroCode}
+  \end{macro}
+
+  Next come the statistical commands.
+  \begin{macro}{\E}
+  \changes{0.1e}{Added \cs{E} command}
+  \changes{0.1f}{Fixed \enquote{Command \cs{E} already defined!} error}
+  \changes{0.2}{Use \cs{operatorname}}
+  Here, we define \cs{E} after the preamble since it may break otherwise.
+\begin{MacroCode}{package}
+\AtBeginDocument{
+  \DeclareDocumentCommand\E{m}{%
+    \ensuremath{\operatorname{E}\left[#1\right]}%
+  }
+}
+\end{MacroCode}
+  \end{macro}
+  The \Macro\P command saves any old \Macro\given command, replacing
+  it locally with the new \Macro\given command provided by the package.
+  \begin{macro}{\P}
+  \changes{0.1e}{Added \cs{P} command}
+  \changes{0.2}{Use \cs{operatorname}, use \cs{cs_new_eq:NN} instead of
+                \cs{let}}
+\begin{MacroCode}{package}
+\DeclareDocumentCommand\P{m}{%
+  \ensuremath{\operatorname{P}%
+    \mkern-1.5mu\left(%
+    \cs_set_eq:NN\__skmath_saved_given:\given%
+\end{MacroCode}
+  \begin{macro}{\given}
+  \changes{0.1e}{Added \cs{given} command}
+\begin{MacroCode}{package}
+    \DeclareDocumentCommand\given{}{\mid}%
+\end{MacroCode}
+  \end{macro}
+\begin{MacroCode}{package}
+    #1%
+    \cs_set_eq:NN\given\__skmath_saved_given:%
+    \right)%
+  }%
+}
+\end{MacroCode}
+  \end{macro}
+  \begin{macro}{\var}
+  \changes{0.1e}{Added \cs{var} command}
+  \changes{0.2}{Use \cs{operatorname}}
+\begin{MacroCode}{package}
+  \DeclareDocumentCommand\var{m}{%
+    \ensuremath{\operatorname{Var}\left(#1\right)}%
+  }
+\end{MacroCode}
+  \end{macro}
+  \begin{macro}{\cov}
+  \changes{0.1e}{Added \cs{cov} command}
+  \changes{0.2}{Use \cs{operatorname}}
+\begin{MacroCode}{package}
+  \DeclareDocumentCommand\cov{mm}{%
+    \ensuremath{\operatorname{Cov}\left(#1,#2\right)}%
+  }
+\end{MacroCode}
+  \end{macro}
+  
+  We replace all trigonometric functions and some other
+  common functions with alternatives that take an argument
+  (or optionally, several arguments).
+\begin{MacroCode}{package}
+\cs_new_eq:NN\__skmath_sin:\sin
+\cs_new_eq:NN\__skmath_cos:\cos
+\cs_new_eq:NN\__skmath_tan:\tan
+\cs_new_eq:NN\__skmath_cot:\cot
+\cs_new_eq:NN\__skmath_arcsin:\arcsin
+\cs_new_eq:NN\__skmath_arccos:\arccos
+\cs_new_eq:NN\__skmath_arctan:\arctan
+\cs_new_eq:NN\__skmath_ln:\log
+\cs_new_eq:NN\__skmath_log:\log
+\cs_new_eq:NN\__skmath_exp:\exp
+\cs_new_eq:NN\__skmath_min:\min
+\cs_new_eq:NN\__skmath_max:\max
+\cs_new_eq:NN\__skmath_sup:\sup
+\cs_new_eq:NN\__skmath_inf:\inf
+\end{MacroCode}
+  \begin{macro}{\sin}
+\begin{MacroCode}{package}
+\RenewDocumentCommand\sin{om}{%
+  \IfNoValueTF{#1}
+    {\ensuremath{\__skmath_sin:\left(#2\right)}}
+    {\ensuremath{\__skmath_sin:\c_math_superscript_token{#1}\left(#2\right)}}%
+}
+\end{MacroCode}
+  \end{macro}
+  \begin{macro}{\cos}
+\begin{MacroCode}{package}
+\RenewDocumentCommand\cos{om}{%
+  \IfNoValueTF{#1}
+    {\ensuremath{\__skmath_cos:\left(#2\right)}}
+    {\ensuremath{\__skmath_cos:\c_math_superscript_token{#1}\left(#2\right)}}%
+}
+\end{MacroCode}
+  \end{macro}
+  \begin{macro}{\tan}
+\begin{MacroCode}{package}
+\RenewDocumentCommand\tan{om}{%
+  \IfNoValueTF{#1}
+    {\ensuremath{\__skmath_tan:\left(#2\right)}}
+    {\ensuremath{\__skmath_tan:\c_math_superscript_token{#1}\left(#2\right)}}%
+}
+\end{MacroCode}
+  \end{macro}
+  \begin{macro}{\cot}
+\begin{MacroCode}{package}
+\RenewDocumentCommand\cot{om}{%
+  \IfNoValueTF{#1}
+    {\ensuremath{\__skmath_cot:\left(#2\right)}}
+    {\ensuremath{\__skmath_cot:\c_math_superscript_token{#1}\left(#2\right)}}%
+}
+\end{MacroCode}
+  \end{macro}
+  \begin{macro}{\arcsin}
+\begin{MacroCode}{package}
+\RenewDocumentCommand\arcsin{m}{%
+  \ensuremath{\__skmath_arcsin:\left(#1\right)}%
+}
+\end{MacroCode}
+  \end{macro}
+  \begin{macro}{\arccos}
+\begin{MacroCode}{package}
+\RenewDocumentCommand\arccos{m}{%
+  \ensuremath{\__skmath_arccos:\left(#1\right)}%
+}
+\end{MacroCode}
+  \end{macro}
+  \begin{macro}{\arctan}
+\begin{MacroCode}{package}
+\RenewDocumentCommand\arctan{m}{%
+  \ensuremath{\__skmath_arctan:\left(#1\right)}%
+}
+\end{MacroCode}
+  \end{macro}
+  \begin{macro}{\ln}
+\begin{MacroCode}{package}
+\RenewDocumentCommand\ln{m}{%
+  \ensuremath{\__skmath_ln:\left(#1\right)}%
+}
+\end{MacroCode}
+  \end{macro}
+  \begin{macro}{\log}
+\begin{MacroCode}{package}
+\RenewDocumentCommand\log{om}{%
+  \IfNoValueTF{#1}
+    {\ensuremath{\__skmath_log:\left(#2\right)}}
+    {\ensuremath{\__skmath_log:\c_math_subscript_token{#1}\left(#2\right)}}%
+}
+\end{MacroCode}
+  \end{macro}
+  \begin{macro}{\exp}
+  \changes{0.1b}{Moved to \textsf{xparse} command definition}
+\begin{MacroCode}{package}
+\RenewDocumentCommand\exp{m}{\ensuremath{\mathchoice%
+  {e\c_math_superscript_token{#1}}%
+  {\__skmath_exp:\left(#1\right)}%
+  {\__skmath_exp:\left(#1\right)}%
+  {\__skmath_exp:\left(#1\right)}%
+}}
+\end{MacroCode}
+  \end{macro}
+  \begin{macro*}{\__skmath_minmax_backend:nnn}
+\begin{MacroCode}{package}
+\cs_new_nopar:Npn\__skmath_minmax_backend:nnnn#1#2#3#4{%
+  \use:c{__skmath_#1:}
+  \IfNoValueF{#3}{
+    \c_math_subscript_token{
+      \mathchoice{\mathclap{#3}}{#3}{#3}{#3}
+    }
+  }
+  \IfBooleanTF{#2}{#4}{\left\{#4\right\}}
+}
+\end{MacroCode}
+  \end{macro*}
+  \begin{macro}{\min}
+\begin{MacroCode}{package}
+\RenewDocumentCommand\min{som}{%
+  \ensuremath{\__skmath_minmax_backend:nnnn{min}{#1}{#2}{#3}}
+}
+\end{MacroCode}
+  \end{macro}
+  \begin{macro}{\argmin}
+\begin{MacroCode}{package}
+\NewDocumentCommand\argmin{som}{%
+  \ensuremath{\arg\__skmath_minmax_backend:nnnn{min}{#1}{#2}{#3}}
+}
+\end{MacroCode}
+  \end{macro}
+  \begin{macro}{\max}
+\begin{MacroCode}{package}
+\RenewDocumentCommand\max{som}{%
+  \ensuremath{\__skmath_max_minbackend:nnnn{max}{#1}{#2}{#3}}
+}
+\end{MacroCode}
+  \end{macro}
+  \begin{macro}{\argmax}
+\begin{MacroCode}{package}
+\NewDocumentCommand\argmax{som}{%
+  \ensuremath{\arg\__skmath_minmax_backend:nnnn{max}{#1}{#2}{#3}}
+}
+\end{MacroCode}
+  \end{macro}
+  \begin{macro}{\sup}
+\begin{MacroCode}{package}
+\RenewDocumentCommand\sup{som}{%
+  \ensuremath{\__skmath_minmax_backend:nnnn{sup}{#1}{#2}{#3}}
+}
+\end{MacroCode}
+  \end{macro}
+  \begin{macro}{\inf}
+\begin{MacroCode}{package}
+\RenewDocumentCommand\inf{som}{%
+  \ensuremath{\__skmath_minmax_backend:nnnn{inf}{#1}{#2}{#3}}
+}
+\end{MacroCode}
+  \end{macro}
+
+  The fraction command is modified to improve typesetting.
+  \begin{macro}{\frac}
+  \changes{0.1b}{Moved to \textsf{xparse} command definition}
+\begin{MacroCode}{package}
+\RenewDocumentCommand\frac{mm}{\genfrac{}{}{}{}%
+             {\displaystyle #1}{\displaystyle #2}}
+\end{MacroCode}
+  \end{macro}
+ 
+  We introduce a couple of helper macros for the \cs{pd} macro backend.
+  \begin{macro*}{\__skmath_if_numerical_p:n}
+  \begin{macro*}{\__skmath_if_numerical:nT}
+  \begin{macro*}{\__skmath_if_numerical:nF}
+  \begin{macro*}{\__skmath_if_numerical:nTF}[1]
+    {Token or string}
+\begin{MacroCode}{package}
+\prg_new_conditional:Npnn\__skmath_if_numerical:n#1{p,T,F,TF}{
+  \str_case_x:nnn{#1}{
+    {0}{\prg_return_true:}
+    {1}{\prg_return_true:}
+    {2}{\prg_return_true:}
+    {3}{\prg_return_true:}
+    {4}{\prg_return_true:}
+    {5}{\prg_return_true:}
+    {6}{\prg_return_true:}
+    {7}{\prg_return_true:}
+    {8}{\prg_return_true:}
+    {9}{\prg_return_true:}
+  }{\prg_return_false:}
+}
+\end{MacroCode}
+  \end{macro*}
+  \end{macro*}
+  \end{macro*}
+  \end{macro*}
+  \begin{macro*}{\__skmath_pd_vars_sum:n}[1]
+    {Comma-separated list of variables, possibly with superscripts}
+\begin{MacroCode}{package}
+\cs_new_nopar:Npn\__skmath_pd_vars_sum:n#1{
+  \tl_clear:N\l_tmpa_tl
+  \int_zero:N\l_tmpa_int
+  \bool_set_true:N\l_tmpa_bool
+  \clist_set:Nn\l_tmpa_clist{#1}
+  \clist_map_inline:Nn\l_tmpa_clist{
+    \seq_set_split:Nnn\l_tmpa_seq{^}{##1}
+    \int_compare:nT{\seq_count:N\l_tmpa_seq<\c_two}{
+      \seq_put_right:Nn\l_tmpa_seq{1}
+    }
+    \seq_get_right:NN\l_tmpa_seq\l_tmpb_tl
+    \__skmath_if_numerical:nTF{\tl_use:N\l_tmpb_tl}{
+      \int_add:Nn\l_tmpa_int{\tl_use:N\l_tmpb_tl}
+    }{
+      \bool_set_false:N\l_tmpa_bool
+      \tl_if_empty:NF\l_tmpa_tl{\tl_put_right:Nn\l_tmpa_tl{+}}
+      \tl_put_right:Nx\l_tmpa_tl{\tl_use:N\l_tmpb_tl}
+    }
+  }
+  \int_compare:nT{\l_tmpa_int>\c_zero}{\int_use:N\l_tmpa_int}
+  \bool_if:NF\l_tmpa_bool{
+    \int_compare:nT{\l_tmpa_int>\c_zero}{+}
+    \tl_use:N\l_tmpa_tl
+  }
+}
+\end{MacroCode}
+  \end{macro*}
+  \begin{macro*}{\__skmath_pd_if_vars_sum_above_one_p:n}
+  \begin{macro*}{\__skmath_pd_if_vars_sum_above_one:nT}
+  \begin{macro*}{\__skmath_pd_if_vars_sum_above_one:nF}
+  \begin{macro*}{\__skmath_pd_if_vars_sum_above_one:nTF}[1]
+    {Comma-separated list of variables, possibly with superscripts}
+\begin{MacroCode}{package}
+\prg_new_conditional:Npnn\__skmath_pd_if_vars_sum_above_one:n#1{p,T,F,TF}{
+  \clist_set:Nn\l_tmpa_clist{#1}
+  \int_compare:nTF{\clist_count:N\l_tmpa_clist>\c_one}{
+    \prg_return_true:
+  }{
+    \clist_pop:NN\l_tmpa_clist\l_tmpa_tl
+    \seq_set_split:NnV\l_tmpa_seq{^}{\l_tmpa_tl}
+    \int_compare:nTF{\seq_count:N\l_tmpa_seq<\c_two}{
+      \prg_return_false:
+    }{
+      \prg_return_true:
+    }
+  }
+}
+\end{MacroCode}
+  \end{macro*}
+  \end{macro*}
+  \end{macro*}
+  \end{macro*}
+  \begin{macro*}{\__skmath_pd_superscript_vars:n}[1]
+    {Comma-separated list of variables, possibly with superscripts}
+\begin{MacroCode}{package}
+\cs_new_nopar:Npn\__skmath_pd_superscript_vars:n#1{
+  \clist_set:Nn\l_tmpa_clist{#1}
+  \clist_map_inline:Nn\l_tmpa_clist{
+    \partial
+    \seq_set_split:Nnn\l_tmpa_seq{^}{##1}
+    \seq_pop:NN\l_tmpa_seq\l_tmpb_tl
+    \tl_use:N\l_tmpb_tl
+    \int_compare:nT{\seq_count:N\l_tmpa_seq>0}{
+      \seq_pop:NN\l_tmpa_seq\l_tmpb_tl
+      \c_math_superscript_token
+      \tl_use:N\l_tmpb_tl
+    }
+  }
+}
+\end{MacroCode}
+  \end{macro*}
+  \begin{macro*}{\__skmath_pd_subscript_vars:n}[1]
+    {Comma-separated list of variables, possibly with superscripts}
+\begin{MacroCode}{package}
+\cs_new_nopar:Npn\__skmath_pd_subscript_vars:n#1{
+  \clist_set:Nn\l_tmpa_clist{#1}
+  \clist_map_inline:Nn\l_tmpa_clist{
+    \seq_set_split:Nnn\l_tmpa_seq{^}{##1}
+    \seq_pop:NN\l_tmpa_seq\l_tmpa_tl
+    \int_set:Nn\l_tmpa_int{\c_one}
+    \int_compare:nT{\seq_count:N\l_tmpa_seq>\c_zero}{
+      \seq_pop:NN\l_tmpa_seq\l_tmpb_tl
+      \int_set:Nn\l_tmpa_int{\tl_use:N\l_tmpb_tl}
+    }
+    \prg_replicate:nn{\l_tmpa_int}{\tl_use:N\l_tmpa_tl}
+  }
+}
+\end{MacroCode}
+  \end{macro*}
+  \begin{macro*}{\__skmath_pd_fraction:nn}[2]
+    {Tokens representing a mathematical function}
+    {Comma-separated list of variables, possibly with superscripts}
+\begin{MacroCode}{package}
+\cs_new_nopar:Npn\__skmath_pd_fraction:nn#1#2{
+  \frac{
+    \partial
+    \__skmath_pd_if_vars_sum_above_one:nT{#2}{
+      \c_math_superscript_token{\__skmath_pd_vars_sum:n{#2}}
+    }
+    {#1}
+  }{
+    \__skmath_pd_superscript_vars:n{#2}
+  }
+}
+\end{MacroCode}
+  \end{macro*}
+  \begin{macro*}{\__skmath_pd_subscript:nn}[2]
+    {Tokens representing a mathematical function}
+    {Comma-separated list of variables, possibly with superscripts}
+\begin{MacroCode}{package}
+\cs_new_nopar:Npn\__skmath_pd_subscript:nn#1#2{
+  {#1}\c_math_subscript_token{
+    \__skmath_pd_subscript_vars:n{#2}
+  }
+}
+\end{MacroCode}
+  \end{macro*}
+
+  Definition of \cs{bar}, \cs{pd} and \cs{d} is deferred until after all
+  packages are loaded to avoid collisions with other packages.
+\begin{MacroCode}{package}
+\AtBeginDocument{%
+\end{MacroCode}
+  The \cs{bar} command is modified to impove typesetting.
+  \begin{macro}{\bar}
+  \changes{0.1b}{Added \cs{bar} replacement}
+  \changes{0.1h}{Wrap in \cs{AtBeginDocument}}
+\begin{MacroCode}{package}
+\DeclareDocumentCommand\bar{m}{%
+    \ensuremath{\mkern 1.5mu\overline{\mkern-1.5mu{#1}\mkern-1.5mu}\mkern 1.5mu}}
+\end{MacroCode}
+  \end{macro}
+
+  This is the partial derivative macro, but most of the functionality
+  was defined as private macros earlier.
+  \begin{macro}{\pd}[3]
+    {Boolean distinguishing between starred and unstarred variant}
+    {Tokens representing a mathematical function}
+    {Comma-separated list of variables, possibly with superscripts}
+\begin{MacroCode}{package}
+\DeclareDocumentCommand\pd{smm}{
+  \ensuremath{  
+    \IfBooleanTF{#1}
+      {\__skmath_pd_subscript:nn{#2}{#3}}
+      {\__skmath_pd_fraction:nn{#2}{#3}}
+  }
+}
+\end{MacroCode}
+  \end{macro}
+
+  We introduce a command to typeset the differential part
+  of integrals, shamefully stolen from an answer on \TeX.SE.
+  \begin{macro}{\d}
+  \changes{0.1a}{Fixed obtuse errors}
+  \changes{0.1b}{Moved to \textsf{xparse} command definition}
+  \changes{0.2}{Use \cs{peek_meaning_ignore_spaces:NT} instead of \cs{@ifnextchar}}
+\begin{MacroCode}{package}
+\DeclareDocumentCommand\d{m}{\ensuremath{\,\mathrm{d}#1%
+                              \peek_meaning_ignore_spaces:NT\d{\!}}}
+\end{MacroCode}
+  \end{macro}
+\begin{MacroCode}{package}
+}
+\end{MacroCode}
+ 
+  Finally, we define a nicer way to denote vectors.
+  \begin{macro}{\vec}
+  \changes{0.2}{Use \cs{cs_new_eq:NN} instead of \cs{let}}
+\begin{MacroCode}{package}
+\cs_set_eq:NN\vec\vectorsym
+\end{MacroCode}
+  \end{macro}
+
+  We end by declaring an option.
+\begin{MacroCode}{package}
+\keys_define:nn{skmath}{
+  commonsets .code:n = 
+    {\__skmath_define_common_sets:}
+}
+\ProcessKeysOptions{skmath}
+\end{MacroCode}
+
+\begin{MacroCode}{package}
+\endinput
+\end{MacroCode}
+
+    \Finale
+    \section{Installation}
+    The easiest way to install this package is using the package
+    manager provided by your \LaTeX\ installation if such a program
+    is available. Failing that, provided you have obtained the package
+    source (\file{skmath.tex} and \file{Makefile}) from either CTAN
+    or Github, running \texttt{make install} inside the source directory
+    works well. This will extract the documentation and code from
+    \file{skmath.tex}, install all files into the TDS tree at
+    \texttt{TEXMFHOME} and run \texttt{mktexlsr}.
+
+    If you want to extract code and documentation without installing
+    the package, run \texttt{make all} instead. If you insist on not
+    using \texttt{make}, remember that packages distributed using
+    \pkg{skdoc} must be extracted using \texttt{pdflatex}, \emph{not}
+    \texttt{tex} or \texttt{latex}.
+
+    \PrintChanges
+    \PrintIndex
+\end{document}