using setf with assoc

I noticed that (setf (assoc key alist) newvalue) fails,
and in sbcl I get the following warning:
; The function (SETF ASSOC) is undefined, and its name is reserved by
ANSI CL so
; that even if it were defined later, the code doing so would not be
portable.

Does anyone know why this is not implemented? Is it because noone
could
agree about what it would do? Or does it really not make sense
in the first place?

.