Exercise 4.74: Alyssa P. Hacker proposes to use
a simpler version of stream-flatmap in negate, lisp-value,
and find-assertions.  She observes that the procedure that is mapped
over the frame stream in these cases always produces either the empty stream or
a singleton stream, so no interleaving is needed when combining these streams.


 Fill in the missing expressions in Alyssa’s program.


(define (simple-stream-flatmap proc s)
  (simple-flatten (stream-map proc s)))

(define (simple-flatten stream)
  (stream-map ⟨??⟩
              (stream-filter ⟨??⟩ 
                             stream)))

 Does the query system’s behavior change if we change it in this way?