Arc's 'defset' is similar to CL's 'get-setf-method'. These functions return a list of information to get or set a 'generalized variable'. When the macro '=' is invoked, it uses this information to generate the code. I guess you could write something like CL's 'defsetf' in Arc (maybe 'def='?). In CLTL2 Common Lisp, this 'defsetf' can be expressed as (defun (setf ...) ...), which seems more natural.
Take a look at PG's book 'On Lisp', chapter 12, pages 171 and 178...
