instrumenter.tex

\RequirePackage{xparse}

\ExplSyntaxOn

\iow_new:N \l_csgillespie_iow

% #1: sectioning command name as a single token
% #2: character tokens representing the command name, without the backslash
\cs_new_protected:Npn \csgillespie_instrument_sec_cmd:Nn #1#2
  {
    % Save the original sectioning command
    \cs_gset_eq:cN { g_csgillespie_#2_orig: } #1

    \RenewDocumentCommand #1 { s O{##3} m }
      {
        \iow_now:Nn \l_csgillespie_iow { #2 ; ##2 ; ##3 }

        \IfBooleanTF {##1}
          { \use:c { g_csgillespie_#2_orig: } * {##3} }
          { \use:c { g_csgillespie_#2_orig: } [##2] {##3} }
      }
  }

\cs_generate_variant:Nn \csgillespie_instrument_sec_cmd:Nn { c }

% #1: character tokens representing the command name, without the backslash
\cs_new_protected:Npn \csgillespie_instrument_sec_cmd_ifexists:n #1
  {
    \cs_if_exist:cT {#1}
      { \csgillespie_instrument_sec_cmd:cn {#1} {#1} }
  }

\AtBeginDocument
  {
    \iow_open:Nn \l_csgillespie_iow { sectioning.csv }
    \clist_map_inline:nn
      {
        chapter, url
      }
      { \csgillespie_instrument_sec_cmd_ifexists:n {#1} }
  }

\AtEndDocument { \iow_close:N \l_csgillespie_iow }

\ExplSyntaxOff
\endinput