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 ())))