>     * lisp/emacs-lisp/advice.el (ad-preactivate-advice):
>     Avoid setting the function definition of nil.
>     This was happening during bootstrap of org-compat.el,
>     apparently due to eager macro expansion of code behind
>     a (featurep 'xemacs) test.

Really, I think this business of "disallow fset of nil" is a big waste
of time and will just lead to more pain than gain.

There are umpteen different ways for the user to shoot himself in the
foot.  This one is not even fatal.

Should we also disallow (fset 'car nil)?  How 'bout (fset 'car #'cdr)?