letrec

Isn't
(letrec ([is-even? (lambda (n)
                    (or (zero? n)
                        (is-odd? (sub1 n))))]
         [is-odd? (lambda (n)
                   (and (not (zero? n))
                        (is-even? (sub1 n))))])
        (is-odd? 11))

equivalent to...

(let
    ([is-even? nil]
     [is-odd? nil])
    (set! is-even? (lambda (n)
                    (or (zero? n)
                        (is-odd? (sub1 n)))))
    (set! is-odd? (lambda (n)
                   (and (not (zero? n))
                        (is-even? (sub1 n)))))
    (is-odd? 11))

>>14
What are delimited continuations?
And there is a much better explanation http://community.schemewiki.org/?composable-continuations-tutorial

it also has an example, how delimited continuations can free your code from a crapload of ugly boilerplate.