--- /dev/null
+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 ())))
+