Exercise 4.75: Implement for the query language
a new special form called unique.  Unique should succeed if there
is precisely one item in the data base satisfying a specified query.  For
example,


(unique (job ?x (computer wizard)))

should print the one-item stream


(unique (job (Bitdiddle Ben)
             (computer wizard)))

since Ben is the only computer wizard, and


(unique (job ?x (computer programmer)))

should print the empty stream, since there is more than one computer
programmer.  Moreover,


(and (job ?x ?j) 
     (unique (job ?anyone ?j)))

should list all the jobs that are filled by only one person, and the people who
fill them.

There are two parts to implementing unique.  The first is to write a
procedure that handles this special form, and the second is to make
qeval dispatch to that procedure.  The second part is trivial, since
qeval does its dispatching in a data-directed way.  If your procedure is
called uniquely-asserted, all you need to do is


(put 'unique 'qeval uniquely-asserted)

and qeval will dispatch to this procedure for every query whose
type (car) is the symbol unique.

The real problem is to write the procedure uniquely-asserted.  This
should take as input the contents (cdr) of the unique
query, together with a stream of frames.  For each frame in the stream, it
should use qeval to find the stream of all extensions to the frame that
satisfy the given query.  Any stream that does not have exactly one item in it
should be eliminated.  The remaining streams should be passed back to be
accumulated into one big stream that is the result of the unique query.
This is similar to the implementation of the not special form.

Test your implementation by forming a query that lists all people who supervise
precisely one person.