(define (triples s t u)
  (cons-stream (list (stream-car s) (stream-car t) (stream-car u))
               (interleave
                (stream-map (lambda (x) (cons (stream-car s) x))
                            (stream-cdr (pairs t u)))
                (triples (stream-cdr s)
                         (stream-cdr t)
                         (stream-cdr u)))))
(define (py-numbers)
  (define (square x) (* x x))
  (define numbers (triples integers integers integers))
  (stream-filter (lambda (x) 
                   (= (square (caddr x)) 
                      (+ (square (car x)) (square (cadr x)))))
                 numbers))