Hello,
thanks for the quick help!
Quoting Ville Skyttä <scop(a)xemacs.org>:
If you really want to support older versions of the XEmacs
regexp-opt,
I'd say checking the number of arguments would be the best way at the
moment (the `shy' argument is in the old XEmacs regexp-opt, but not in
the synced up one).
OK, the best seems to drop compatibility with older versions
and upgrade the respective package.
Cheers, WB