rplac1 - replace 1st occurrence in nested list

rplac1.scm

(define (rplac1 xs a b)
  (let g ((xs xs) (f #f) (k (lambda (x y) x)))         ; http://ideone.com/AbWKxS
    (cond
      (f          (k xs f))                 ; shortcut!
      ((null? xs) (k xs f))                            ; http://stackoverflow.com/q/16550176/849891
      ((not (pair? xs))                                ; http://stackoverflow.com/q/16444290/849891
        (if (eq? xs a) (k b #t) (k xs f)))  ; not f!
      (else 
        (g (car xs)   f   (lambda (x f) 
        (g (cdr xs)   f   (lambda (y f) 
        (if (not f) 
            (k xs f) 
            (k (cons x y) f))))))))))