% SPLIT.TEX

% Marek Ry\'cko

% Macro \split (and auxiliary macros)
% for splitting token sequences
% Macro \hdtl

% version 0.0 24.07.1993
% version 0.1 31.12.94
% version 0.2 1.5.1996
%   -- english description

\def\hdtl
    #1% inp. t.s. (nonempty, poss blanks)
    #2% outp. -- head
    #3% outp. -- tail
    {\let\reshead=#2% synonims for token-register names
    \let\restail=#3%
    \expandafter\gethead\the#1\endoftokens}
\def\gethead
    {\futurelet\thehead\analysehead}
\def\analysehead
    {\ifcat \noexpand\thehead \space
        \let\followby=\followbyspace
    \else
        \let\followby=\followbynospace
    \fi
    \followby}
\def\followbyspace
    {\reshead={ }% space token
    \afterassignment\gettail
    \let\swallow= }% swallowing the head-space
\def\gettail
     #1\endoftokens
     {\restail={#1}}
\def\followbynospace
     #1#2\endoftokens
     {\reshead={#1}%
     \restail={#2}}
\def\split
    #1% input macro name (should expand to nonemty token sequence)
    #2% macro head (name)
    #3% macro tail (name)
    {%
    % define
    %   #2 as the first token (with nested braces) of #1
    %   #3 as the tail        (with nested braces) of #1
    \expandafter\splithdtl\expandafter{#1}%
    \expandafter\def\expandafter\splittest\expandafter{\the\splithdtl}%
    \ifx\splittest\empty
        \gdef#2{}%
        \gdef#3{}%
    \else
        \hdtl\splithdtl\splithd\splittl
        \expandafter\gdef\expandafter#2\expandafter{\the\splithd}%
        \expandafter\gdef\expandafter#3\expandafter{\the\splittl}%
    \fi
    }
\newtoks\splithdtl
\newtoks\splithd
\newtoks\splittl