[texhax] Mathop with mathop spacing?

[Michael Barr]

The following answers the question I raised originally, although not the 
most general question.  It is not exactly Uwe Lu\"ck's suggestion but what 
he said made me think of it:

\documentclass{article}
\makeatletter
\def\xsub_#1{\mathrel{\mathop{\times}\limits_{#1}}}
\def\x{\@ifnextchar_{\xsub}{\times}}
\makeatother
\begin{document}
 $$A\x B$$
 $$A\x_CB$$
 $$A\x _C B$$
 $$A\x _{CD} B$$
\end{document}