Philip Taylor (RHBNC) writes:
 > (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).

I think it is better - at the moment I can use \def in my document
without doing anything special.  If I want to use a command \@def I
have to go to a little trouble - not much trouble, but at least I know
that there might be consequences of my actions!

Renaming it \tex/def achieves the same result because / will have its
normal catcode while processing a document.