;; replace once, with two continuations - success and failure
(define (rplac1_2 xs a b)
  (let g ((xs xs) (s (lambda (x) x)) (f (lambda () xs)))
    (cond
      ((null? xs) (f))
      ((not (pair? xs))
        (if (eq? xs a) (s b) (f)))
      (else
        (g (car xs)
           (lambda (x) ; succeded!
             (s (cons x (cdr xs))))
           (lambda () ; failed (nothing to replace yet, keep trying)
             (g (cdr xs)
                (lambda (y) ; succeeded!
                  (s (cons (car xs) y)))
                 f)))))))   ; none replaced
 
(display (rplac1_2 '((((a 2) 3 4) a) 6) 'a 'b)) (newline)
(display (rplac1_2 '((((c 2) 3 4) a) 6) 'a 'b)) (newline)
(display (rplac1_2 '((((c 2) 3 a) a) 6) 'a 'b)) (newline)
 
;; Success	time: 0.03 memory: 4176 signal:0
;; ((((b 2) 3 4) a) 6)
;; ((((c 2) 3 4) b) 6)
;; ((((c 2) 3 b) a) 6)

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