>>   I guess there will be a command called \tex_def or \tex/def or something,
>> which one can use.

Indeed, that was my very point : David was suggesting that \def could
be made inaccessible by the format; I argued that all the while TeX lacks
the equivalent of PostScript's "bind", primitives which are used by
maos defined in the format source and which must be accessible to
the user code can never be made totally inaccessible.  You can "hide"
them but you can't remove them, so why bother even to hide them if an astute
programmer can work his way around the hiding mechanism?

** Phil.

(the earlier proposal to use commercial-at is just one way of hiding such
things, neithe better nor worse (in this context) than any other mechanism).