\input plainpkg
\ProvidesPackage{stacklet}[2012/11/07 v0.3a private letters (UL)]
%%
%% Copyright (C) 2012 Uwe Lueck,
%% http://www.contact-ednotes.sty.de.vu
%% -- author-maintained in the sense of LPPL below --
%%
%% This file can be redistributed and/or modified under
%% the terms of the LaTeX Project Public License; either
%% version 1.3c of the License, or any later version.
%% The latest version of this license is in
%%     http://www.latex-project.org/lppl.txt
%% There is NO WARRANTY (actually somewhat experimental).
%%
%% Please report bugs, problems, and suggestions via
%%
%%   http://www.contact-ednotes.sty.de.vu
%%
%%  == Usage                    ==
%% === Installing and Calling   ===
%% The file `stacklet.sty' is provided ready, 
%% installation only requires
%% putting it somewhere where \TeX\ finds it
%% (which may need updating the filename data
%%  base).\urlfoot{ukfaqref}{inst-wlcf} 
%% However, the file `plainpkg.tex' must be installed likewise.
%% 
%% \emph{With} \LaTeX, the file should be loaded by `\RequirePackage{stacklet}'
%% or `\usepackage{stacklet}'. 
%% 
%% \emph{Without} \LaTeX, both `\input stacklet.sty' and 
%% `\input plainpkg' load `stacklet.sty'.
%%
%% \filbreak                                        %% 2012/11/07
%% === Commands and Syntax      ===
%% \label{sec:stacklet-cmds}
%% 'stacklet.sty' provides 
%% \[|\PushCatMakeLetter\<char>|
%%   \quad \textit{and}\quad 
%%   |\PopLetterCat\<char>|\]
%% for getting ``private letters" and giving them back
%% their previous category code
%% in package files with or without \LaTeX.
%% As \LaTeX\ has its own stack for `@', there are also 
%% \[|\PushCatMakeLetterAt|
%%   \quad \textit{and}\quad 
%%   |\PopLetterCatAt|\]
%% that care for `@''s category code \emph{without} \LaTeX\ only.
%% 
%%  == The Code                 ==
%% === Name Space               ===
%% Each ``private letter" <char> gets its own stack, 
%% in some name space, determined by |\cat_stack|
%% (`\withcsname' is from 'plainpkg.tex'):
\withcsname\xdef cat_stack\endcsname{%
    \noexpand\string \withcsname\noexpand cat_stack\endcsname
        \noexpand\string}
%% I.e., ?`cat_stack' will expand to 
%% \[?`string'\,?`cat_stack'\,?`string'\]
%% in the notation of the \ctanpkgref{dowith} package
%% documentation.                                       %% 2012/11/06
% \withcsname\show cat_stack\endcsname
%% 
%% === Pushing                  ===
%% |\PushCatMakeLetter\<char>| ...
\xdef\PushCatMakeLetter#1{%
  \noexpand\withcsname
    \withcsname\noexpand pushcat_makeletter\endcsname
      \withcsname\noexpand cat_stack\endcsname#1\endcsname#1}
% \show\PushCatMakeLetter
\withcsname\gdef pushcat_makeletter\endcsname#1#2{%
%% #1 is the stack token, #2 is the ``quoted" character. Pushing~...
    \xdef#1{\the\catcode`#2\relax%
%% ... the new entry. `\relax' separates entries, 
%% braces instead tend to get lost in popping ...
%% If the stack has existed before, 
%% its previous content is appended:
            \ifx#1\relax \else #1\fi}%
%% I thought of storing `\catcode's hexadecimally (without braces) 
%% using \LaTeX's `\hexnumber', but the latter has so many tokens~...
%% Finally rendering <char> a ``letter":
    \catcode`#211\relax}
%% Now we can use a ``private letter stack" for our own package:
\PushCatMakeLetter\_
%%
%% === Popping                  ===
%% |\PopLetterCat\<char>| passes `\<char>', the corresponding 
%% stack token, and the latter's expansion to `\popcat_'. 
%% `\end' serves as argument delimiter for the end of the stack only:
\gdef\PopLetterCat#1{%
  \expandafter\expandafter\expandafter
    \popcat_\csname\cat_stack#1\expandafter\endcsname
      \expandafter \end \csname\cat_stack#1\endcsname#1}
%% `\popcat_' parses the expansion, assigns the old 
%% category code and and stores the reduced stack:
\gdef\popcat_#1\relax#2\end#3#4{\catcode`#4#1\gdef#3{#2}}
%% ... check existence? TODO
%%
%% === No `@' Stack with \LaTeX ===
%% |\PushCatMakeLetterAt| is like `\PushCatMakeLetter\@'
%% except that it has no effect under \LaTeX:
\gdef\PushCatMakeLetterAt{\ifltx\else\PushCatMakeLetter\@\fi}
%% |\PopLetterCatAt| by analogy ...
\gdef\PopLetterCatAt{\ifltx\else\PopLetterCat\@\fi}
%% 
%% === Leaving the Package File ===
%% ... in our new way:
\PopLetterCat\_
\endinput
%%
%% === VERSION HISTORY ===

v0.1   2012/08/24   started
       2012/08/25   completed
       2012/08/26   extending doc.; \def\withcsname removed
v0.2   2012/08/26   \with_catstack containing \endcsname and with
                    three popping macros replaced by \csname 
                    content \cat_stack, cf. memory.tex; 
                    restructured
       2012/08/27   \PushCatMakeLetterAt fixed
v0.3   2012/08/27   def.s global
v0.3a  2012/11/06   doc.: "documentation"
       2012/11/07   \filbreak