[cstex] \def v \caption

[Jan Holub]

Zdravim,

uz nekolik hodin si lamu hlavu nad tim, proc me LaTeXove makro, ktere 
pouzivam jiz velmi dlouho, nefunguje v \caption v prostredi figure. Jinde 
funguje. Vubec netusim, ceho se chytit. Nemate nekdo nejaky napad?

Kompletni zdrojak a chybu uvadim dole. Chci, aby se za fce uvedly 
zavorky a v nich parametr nebo bez zavorek, kdyz zadny parametr 
neni zadan. Pouzivam tetex.

Predem diky za rady.
Jan Holub

-------------------------------------------------------------------------
\documentclass[a4paper,11pt]{article}
\def\ecko#1{\ensuremath{\varepsilon\textit{fce}}%
\def\XXbb{#1}\ifx\XXbb\empty\relax\else(#1)\fi}

\begin{document}

\section{Introduction}

%\ecko{}

\begin{figure}
%\ecko{}
\caption{aaa \ecko{a} aaa}
\end{figure}

\end{document}

-------------------------------------------------------------------------
Document Class: article 2004/02/16 v1.4f Standard LaTeX document class
(/usr/share/texmf/tex/latex/base/size11.clo)) (./pokus.aux)
! Undefined control sequence.
\ecko ...th {\varepsilon \textit {fce}}\def \XXbb
                                                   {#1}\ifx \XXbb \empty 
\rel...
l.13 \caption{aaa \ecko{a} aaa}
-------------------------------------------------------------------------