+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)
+