> Many modern schemes return a special value labeled "undefined",
> which it is an error to refer to or store.  But I think MITscheme
> still does the old-school value returns.

In what scheme is it an error to store the undefined value?
I've just tried
(define undefined (if #f #f))
in Gauche, MzScheme and Sisc and none signalled an error.

--
Grzegorz