Exercise 3.68.  Louis Reasoner thinks that building a stream of pairs 
from three parts is unnecessarily complicated. Instead of separating the 
pair (S0,T0) from the rest of the pairs in the first row, he proposes to 
work with the whole first row, as follows:

(define (pairs s t)
  (interleave
   (stream-map (lambda (x) (list (stream-car s) x))
               t)
   (pairs (stream-cdr s) (stream-cdr t))))

Does this work? Consider what happens if we evaluate (pairs integers 
integers) using Louis's definition of pairs.

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Mathematically Louis's version gives a correct result, however, because 
of evaluation order, (pairs (stream-cdr s) (stream-cdr t)) will be 
evaluated before interleave is called.

As a result, each call to pairs will generate a new call to pairs, and 
no result will ever be returned.

This is avoided in the original pairs implementation by using 
cons-stream.  Alternatively, if (interleave) was implemented as a 
special form like if or cond, which only evaluated the second argument 
once the second result element was requested, then this would work.