Exercise 4.31: The approach taken in this
section is somewhat unpleasant, because it makes an incompatible change to
Scheme.  It might be nicer to implement lazy evaluation as an

upward-compatible extension, that is, so that ordinary Scheme
programs will work as before.  We can do this by extending the syntax of
procedure declarations to let the user control whether or not arguments are to
be delayed.  While we’re at it, we may as well also give the user the choice
between delaying with and without memoization.  For example, the definition


(define (f a (b lazy) c (d lazy-memo))
  …)

would define f to be a procedure of four arguments, where the first and
third arguments are evaluated when the procedure is called, the second argument
is delayed, and the fourth argument is both delayed and memoized.  Thus,
ordinary procedure definitions will produce the same behavior as ordinary
Scheme, while adding the lazy-memo declaration to each parameter of
every compound procedure will produce the behavior of the lazy evaluator
defined in this section. Design and implement the changes required to produce
such an extension to Scheme.  You will have to implement new syntax procedures
to handle the new syntax for define.  You must also arrange for
eval or apply to determine when arguments are to be delayed, and
to force or delay arguments accordingly, and you must arrange for forcing to
memoize or not, as appropriate.