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 This is a same question to exercise 4.71. 


it will end up with an infinite loop when the stream is infinite and we won't get any results. The two arguments (stream-car stream) and (flatten-stream (stream-cdr stream)) will be evaluated first, before the interpreter apply interleave to them. So the flatten-stream will try to get all the elements in the stream, and will never stop until the stream is null.

With a finite stream, it will function well, but it has enumerated all the elements in the stream, which means the usage of stream will be meaningless.

 (define (flatten-stream stream) 
   (if (stream-null? stream) 
        (stream-car stream) 
        (flatten-stream (stream-cdr stream)))))