R4RS 6.7  (string-set! string integer char)  ==>  unspecific

INTEGER must be a valid index of STRING. STRING-SET! stores CHAR
in the element at the INTEGER'th position of STRING and returns an
unspecified value.

(define (f) (make-string 3 #\*))
(define (g) "***")
(string-set! (f) 0 #\?)                          ==>  unspecified
(string-set! (g) 0 #\?)                          ==>  error
(string-set! (symbol->string 'immutable) 0 #\?)  ==>  error
