Thanks again. 

I've tweaked and commented the end of Mathias's example:

(module+ test
  (check-exn exn:fail:contract? (lambda () (f 1))) ; violates contract by 
returning non-integer
  (check-exn exn:fail:contract? (lambda () (f 'a)))) ; violates contract by 
being called with a non-integer

and "thunking" makes my example work as I wanted:

 (lambda () 
    (pairwise-stream (stream-cons 1 empty-stream)))))

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