Although I do like the idea of a record/object syntax for Maxima, it
seems to me very premature to hardwire this @-syntax into mset.
If we must have something like this in the code sooner rather than
later (which I'm not convinced of), let's at least make it
controllable, e.g.
((assq (caar x) $mset_extension_operator)
(funcall (cdr (assq (caar x) $mset_extension_operator)) x y))
-s