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.

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Unless can be trivially implemented as a derived expression by simply 
reversing the arguments and using if.

The earlier stream programming simulation exercises provide an example 
where unless as a procedure would be convenient.  If a program produces 
three streams, one of which is a binary signal that determines which of 
the other two streams will be output, this could be implemented as

(stream-map unless a b c),

if unless were an ordinary procedure.

This could still be implemented using a lambda in ordinary Scheme:

(stream-map (lambda (a b c) (unless a b c)) a b c).

If the streams are lazy, then the version using lambda does more work, 
and may not always give the same result (unless the language is lazy 
"all the way down").