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))

>>37
The original idea was:
http://www.jwz.org/doc/worse-is-better.html
>Correctness-the design must be correct in all observable aspects. Incorrectness is simply not allowed.
Consistency-the design must not be inconsistent. A design is allowed to be slightly less simple and less complete to avoid inconsistency. Consistency is as important as correctness.
Why Scheme people interpreted it as "undefined behaviour is consistent" is beyond me.