Exercise 4.72.  Why do disjoin and stream-flatmap interleave the streams 
rather than simply append them? Give examples that illustrate why 
interleaving works better. (Hint: Why did we use interleave in section 
3.5.3?)

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As in 3.5.3, when infinite streams are combined, interleaving is the 
only way to ensure that each element of each stream will eventually be 
reached.  If the streams were appended, elements of the second stream 
would never be reached.

It's not clear to me whether infinite streams of results would be 
handled well by the interpreter in any case.

It also seems unlikely that infinite streams would be generated except 
in the case of errors in the database (such as unintentionally recursive 
rules), though it is possible that the implementation could be extended 
to use infinite result streams more usefully.