R4RS 6.3  (set-car! pair object)  ==>  unspecific

Stores OBJECT in the car field of PAIR. The value returned by
SET-CAR! is unspecified.

(define (f) (list 'not-a-constant-list))
(define (g) '(constant-list))

(set-car! (f) 3)  ==>  unspecified
(set-car! (g) 3)  ==>  error
