昇順の整数のリストが複数あったとき、それらの共通部分を抜き出すやつ（わかりにくい）

intersection.tex

\documentclass{article}
\makeatletter %!!!!!!!!!!!!!!!!!!!!!!!!! TeX code BEGIN
\newif\ifxx@ok
\newcommand*\xxLetIntersectionTo[2]{%
  \let\xx@ret\relax
  \@tfor\xx@l:=#2\do{%
    \ifx\xx@ret\relax \edef\xx@ret{\xx@l}%
    \else \xx@intersect@two \fi}%
  \let#1\xx@ret}
\def\xx@head#1#2{\edef\xx@next{\noexpand\xx@head@a#2}\xx@next\relax#1}
\def\xx@head@a#1#2\relax#3{\def#3{#1}}
\def\xx@drop#1{\edef\xx@next{\noexpand\xx@drop@a{#1}}\xx@next#1}
\def\xx@drop@a#1{\expandafter\xx@drop@b\expandafter{\@gobble#1}}
\def\xx@drop@b#1#2{\def#2{#1}}
\def\xx@intersect@two{%
  \let\xx@la\xx@ret \edef\xx@lb{\xx@l}\let\xx@ret\@empty\xx@oktrue
  \@whilesw\ifxx@ok\fi{%
    \ifx\xx@la\@empty \xx@okfalse
    \else\ifx\xx@lb\@empty \xx@okfalse
    \else \xx@head\xx@hda\xx@la \xx@head\xx@hdb\xx@lb
      \ifnum\xx@hda<\xx@hdb \xx@drop\xx@la
      \else\ifnum\xx@hda>\xx@hdb \xx@drop\xx@lb
      \else \edef\xx@ret{\xx@ret{\xx@hda}}\xx@drop\xx@la \xx@drop\xx@lb
      \fi\fi
    \fi\fi}}
\makeatother  %!!!!!!!!!!!!!!!!!!!!!!!!! TeX code END

\def\listA{{1}{2}{3}{4}{5}{6}{8}}
\def\listB{{1}{4}{8}}
\def\listC{{1}{2}{8}}
\xxLetIntersectionTo\listR{\listA\listB\listC}
\show\listR %==>{1}{8}

\stop