REMOVE-FROM-SET
===============

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

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

This function removes an element from 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: >(remove-from-set bonn *capital-cities*)
         >(same-sets * *capital-cities*)
         T

         >(remove-from-set hamburg *cities*)
         #<set-of CITY (...)>
       
         >(set-to-list *)
         (#<CAPITAL-CITY PARIS> #<CITY DORTMUND>
            #<CAPITAL-CITY BRUESSEL> #<CITY BONN>
                #<CAPITAL-CITY BERLIN>)