This function is developed in *The Seasoned Schemer* pp. 84-89. It accepts an atom `a` and a list `lst` and returns `lst` with the leftmost occurrence of `a`, if any, removed. Occurrences of `a` will be found no matter how deeply embedded.
#lang racket
(define (atom? x)
(and (not (pair? x)) (not (null? x))))
(define gamma
(lambda (a lst)
(letrec ([aux (lambda (l k)
(cond
[(null? l) (k 'notfound)]
[(eq? (car l) a) (cdr l)]
[(atom? (car l)) (cons (car l) (aux (cdr l) k))]
[else
; when (car l) exists but isn't an atom, we try to remove a from (car l)
; if we succeed we prepend the result to (cdr l) and stop
(let ([car2 (let/cc k2
; calling k2 with val will bind car2 to val and continue with the (cond ...) block below
(aux (car l) k2))])
(cond
; if a wasn't found in (car l) then prepend (car l) to the result of removing a from (cdr l)
[(eq? car2 'notfound) (cons (car l) (aux (cdr l) k))]
; else a was found in (car l)
[else (cons car2 (cdr l))]))]))]
[lst2 (let/cc k1
; calling k1 with val will bind lst2 to val and continue with the (cond ...) block below
(aux lst k1))])
(cond
; was no atom found in lst?
[(eq? lst2 'notfound) lst]
[else lst2]))))
(gamma 'a '(((a b) ()) (c (d ())))) ; ~~> '(((b) ()) (c (d ())))
(gamma 'a '((() (a b) ()) (c (d ())))) ; ~~> '((() (b) ()) (c (d ())))
(gamma 'a '(() (() (a b) ()) (c (d ())))) ; ~~> '(() (() (b) ()) (c (d ())))
(gamma 'c '((() (a b) ()) (c (d ())))) ; ~~> '((() (a b) ()) ((d ())))
(gamma 'c '(() (() (a b) ()) (c (d ())))) ; ~~> '(() (() (a b) ()) ((d ())))
(gamma 'x '((() (a b) ()) (c (d ())))) ; ~~> '((() (a b) ()) (c (d ())))