number of arg checking.

[GJC at MIT-MC (George J. Carrette)]

FOOBAR!  (defmacro foo (x) `(cdr ,x))

(setf (foo '(a b c) '99))

Returns NIL and causes no error.

Not only does SETF not do wrong-number of argument checking,
it somehow manages to suppress the checking in the
macros which it expands. How is it so clever?
This sure caused me to waste a whole lot of time.
FOOBAR FOOBAR FOOBAR.