;; NEW CONTRACT

(replace-at? u"ab" u1 u"c") ;; Returns (some u"ac")
(replace-at? 0x00112233 u2 0x44) ;; Returns (some 0x00114433)
(replace-at? "abcd" u3 "e") ;; Returns (some "abce")
(replace-at? (list 1) u0 10) ;; Returns (some (10))
(replace-at? (list (list 1) (list 2)) u0 (list 33)) ;; Returns (some ((33) (2)))
(replace-at? (list 1 2) u3 4) ;; Returns none
;;
