Exercise 4.40: In the multiple dwelling problem,
how many sets of assignments are there of people to floors, both before and
after the requirement that floor assignments be distinct?  It is very
inefficient to generate all possible assignments of people to floors and then
leave it to backtracking to eliminate them.  For example, most of the
restrictions depend on only one or two of the person-floor variables, and can
thus be imposed before floors have been selected for all the people.  Write and
demonstrate a much more efficient nondeterministic procedure that solves this
problem based upon generating only those possibilities that are not already
ruled out by previous restrictions.  (Hint: This will require a nest of
let expressions.)