Currently, M-D-M-C signals an error if it's invoked on a char that is
already a dispatch macro character.
Clisp, ECL, and CCL do not signal an error, but instead overwrite the
dispatch-tables of the dispatch char to the default value.
Given that M-D-M-C's Exceptional Situations are "None", I find that
It's also what fits better in my Named-Readtables library because
otherwise redefinition of readtable definitions is a chore.
I committed an appropriate patch into my `tcr-reader-hacking' branch,
but I wanted to ask if such a change would result in any objections.