ADD-TO-SET
==========

syntax: (add-to-set <lisp-type: T>
                    <evaluates-to <set-of A (...)>>
                    [:TEST <lisp-type: function>])

        ==> <set-of A (...)> | <new-set-of A (...)>

This function adds an element to a set.

Warning: type checking may cause an error:
         if all elements of a set have the same
         type, check the first elem. with CAVEAT:
         what about empty set ?!?

example: >(add-to-set bonn *capital-cities*)
         Error: The elementtype restriction 
                is used wrong.

         >(add-to-set hamburg *cities*)
         #<set-of CITY (...)>

         >(set-to-list *)
         (#<CITY HAMBURG> #<CAPITAL-CITY PARIS> 
              #<CITY DORTMUND> #<CAPITAL-CITY 
                   BRUESSEL> #<CITY BONN>
                         #<CAPITAL-CITY BERLIN>)