\documentclass{article}

\makeatletter
% building associative functions. Using \def\disj{\assoc\vee},
% $\disj{a,b,c}$ evaluates to (a\vee b\vee c)
% \bassoc, is a variant of assoc with brackets around
\def\assoc#1#2{\let\@tmpop\relax\@for\@I:=#2\do{\@tmpop\@I\let\@tmpop#1}}
\def\disj{\assoc\times}
\makeatother
\begin{document}
$\disj{a,b,c}$
\end{document}
