Exercise 4.19: Ben Bitdiddle, Alyssa P. Hacker,
and Eva Lu Ator are arguing about the desired result of evaluating the
expression


(let ((a 1))
  (define (f x)
    (define b (+ a x))
    (define a 5)
    (+ a b))
  (f 10))

Ben asserts that the result should be obtained using the sequential rule for
define: b is defined to be 11, then a is defined to be 5,
so the result is 16.  Alyssa objects that mutual recursion requires the
simultaneous scope rule for internal procedure definitions, and that it is
unreasonable to treat procedure names differently from other names.  Thus, she
argues for the mechanism implemented in Exercise 4.16.  This would lead
to a being unassigned at the time that the value for b is to be
computed.  Hence, in Alyssa’s view the procedure should produce an error.  Eva
has a third opinion.  She says that if the definitions of a and b
are truly meant to be simultaneous, then the value 5 for a should be
used in evaluating b.  Hence, in Eva’s view a should be 5,
b should be 15, and the result should be 20.  Which (if any) of these
viewpoints do you support?  Can you devise a way to implement internal
definitions so that they behave as Eva prefers?230