#lang racket

(define (interleave s1 s2)
  (if (stream-null? s1)
      s2
      (cons-stream (stream-car s1)
                   (interleave s2 (stream-cdr s1)))))

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