This function is developed in *The Seasoned Schemer* pp. 55-60. It accepts an atom `a` and a list of atoms `lst`, and returns the part of `lst` following the last occurrence of `a`. If `a` is not in `lst`, it returns `lst` unaltered. #lang racket (define (atom? x) (and (not (pair? x)) (not (null? x)))) (define alpha (lambda (a lst) (let/cc k ; calling k with val will immediately return val from the call to alpha (letrec ([aux (lambda (l) (cond [(null? l) '()] [(eq? (car l) a) ; we abandon any waiting recursive (aux ...) calls, and instead immediately return (aux (cdr l)) ; ...since Scheme is call-by-value, (aux (cdr l)) will be evaluated first, and ; any calls to k therein will come first (and the pending (k ...) here will be abandoned) (k (aux (cdr l)))] [else (cons (car l) (aux (cdr l)))]))]) (aux lst))))) (alpha 'a '(a b c a d e f)) ; ~~> '(d e f) (alpha 'x '(a b c a d e f)) ; ~~> '(a b c a d e f) (alpha 'f '(a b c a d e f)) ; ~~> '() (alpha 'a '(a b c x d e f)) ; ~~> '(b c x d e f)