Exercise 4.26: Ben Bitdiddle and Alyssa
P. Hacker disagree over the importance of lazy evaluation for implementing
things such as unless.  Ben points out that it’s possible to implement
unless in applicative order as a special form.  Alyssa counters that, if
one did that, unless would be merely syntax, not a procedure that could
be used in conjunction with higher-order procedures.  Fill in the details on
both sides of the argument.  Show how to implement unless as a derived
expression (like cond or let), and give an example of a situation
where it might be useful to have unless available as a procedure, rather
than as a special form.