Exercise 3.72.  In a similar way to exercise 3.71  generate a stream of 
all numbers that can be written as the sum of two squares in three 
different ways (showing how they can be so written).

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(define (cube-sum pair)
  (+ (expt (car pair) 3)
     (expt (cadr pair) 3)))

(define pairs-by-cube-sum
  (weighted-pairs integers integers cube-sum))

(define (weight-equal stream weight)
  (define (go stream pair target)
    (let* ((next-pair (stream-car stream))
           (next-weight (weight next-pair)))
      (if (= next-weight target)
          (let* ((third-pair (stream-cadr stream))
                 (third-weight (weight third-pair)))
            (if (= third-weight target)
                (cons-stream (list pair next-pair third-pair)
                             (go (stream-cdr stream) next-pair next-weight))
                (go (stream-cdr stream) next-pair next-weight)))
          (go (stream-cdr stream) next-pair next-weight))))
  (go (stream-cdr stream) (stream-car stream) (weight (stream-car stream))))

(define ramanujan-triples
  (weight-equal pairs-by-cube-sum cube-sum))

The first few are:
((255 414) (228 423) (167 436))
((346 428) (90 492) (11 493))
((408 423) (359 460) (111 522))
((315 525) (198 552) (70 560))