% bxenclose.sty

%% package declaration
\NeedsTeXFormat{LaTeX2e}
\ProvidesPackage{bxenclose}[2016/04/01 v0.2]
\def\bxew@pkgame{bxenclose}

%--------------------------------------- main

%% \bxew@begin@body@hook
% Things to be prepended to the document body.
\let\bxew@begin@body@hook\@empty
%% \bxew@end@body@hook
% Things to be appended to the document body.
\let\bxew@end@body@hook\@empty

% The insertion of \bxew@begin@body@hook is realized by appending
% the token to \document macro. But of course there are dozens of
% packages that do the same thing, and in order to "win the race"
% and put \bxew@begin@body@hook immediately before the actual body
% content, I need to postpone the patching to \document as late as
% possible. Hence comes my tactics, where the patching will be done
% in invocation of "\begin{document}", just before expanding the
% \document macro.

%%<*> \enclosebodywith{<begin>}{<end>}
\@onlypreamble\enclosebodywith
\newcommand\enclosebodywith[2]{%
 \begingroup
    \ifx\bxew@patch@begin\@undefined\else
      \bxew@patch@begin
    \fi
    \toks@\expandafter{\bxew@begin@body@hook}%
    \toks\tw@{#1}%
    \xdef\bxew@begin@body@hook{%
      \the\toks@\the\toks\tw@}%
    \toks@\expandafter{\bxew@end@body@hook}%
    \toks\tw@{#2}%
    \xdef\bxew@end@body@hook{%
      \the\toks\tw@\the\toks@}%
  \endgroup
}

%% \bxew@patch@begin
% Patches to \begin macro. Specifically, it prepends
% "\bxew@begin@hook{#1}" to \begin.
\def\bxew@patch@begin{%
  \global\let\bxew@patch@begin\@undefined % one-shot
  \begingroup
    \toks@\expandafter{\begin{##1}}%
    \edef\bxew@tempa{\gdef\noexpand\begin####1{%
      \noexpand\bxew@begin@hook{####1}\the\toks@}}%
    \bxew@tempa
  \endgroup
}

%% \bxew@unpatch@begin
% Revokes the patch of \bxew@patch@begin. That is, it removes
% "\bxew@begin@hook{#1}" from \begin macro.
\def\bxew@unpatch@begin{%
  \global\let\bxew@unpatch@begin\@undefined % one-shot
  \begingroup
    % check the target
    \long\def\bxew@tempa##1\bxew@begin@hook##2##3\bxew@end{%
      \def\bxew@tempa{##2}}%
    \expandafter\bxew@tempa\begin{}\bxew@begin@hook!\bxew@end
    \ifx\bxew@tempa\@empty % if target is there
      \long\def\bxew@tempa##1\bxew@begin@hook##2{##1}%
      \toks@\expandafter\expandafter\expandafter
       {\expandafter\bxew@tempa\begin{##1}}%
      \edef\bxew@tempa{\gdef\noexpand\begin####1{\the\toks@}}%
      \bxew@tempa
    \fi % if not there, then do nothing
  \endgroup
}

%% \bxew@begin@hook{<envname>}
% The patch attached to \begin. When "\begin{document}"
% is called, this macro patches to \document, before
% \document is expanded.
\def\bxew@begin@hook#1{%
  \begingroup
    \def\bxew@tempa{#1}\def\bxew@tempb{document}%
    \ifx\bxew@tempa\bxew@tempb % if it is \begin{document}
      % just in case unpatching failed
      \global\let\bxew@begin@hook\@gobble % neutralize
      \bxew@unpatch@begin
      \bxew@patch@document
    \fi
  \endgroup
}

%% \bxew@patch@document
% Appends something and \bxew@begin@body@hook to \document macro.
\def\bxew@patch@document{%
  \begingroup
    \global\let\bxew@patch@document\@undefined % one-shot
    \g@addto@macro\document{%
      \bxew@patch@enddocument
      % and \bxew@begin@body@hook will come last
      \let\reserved@a\bxew@begin@body@hook
      \global\let\bxew@begin@body@hook\@undefined
      \reserved@a}%
  \endgroup
}

% Just like \bxew@begin@body@hook, prepending \bxew@end@body@hook
% to \enddocument need to be deferred as much as possible. So I
% decided to patch \enddocument just before executing the
% \bxew@begin@body@hook macro.

%% \bxew@patch@enddocument
% Prepends \bxew@end@body@hook to \enddocument.
\def\bxew@patch@enddocument{%
  \global\let\bxew@patch@enddocument\@undefined % one-shot
  \begingroup
    \toks@\expandafter{\enddocument}%
    \toks\tw@{\bxew@end@body@hook
      \global\let\bxew@end@body@hook\@undefined}%
    \xdef\enddocument{\the\toks\tw@\the\toks@}%
  \endgroup
}

%--------------------------------------- done
\endinput
%% EOF