IS-ELEMENT-OF-SET
=================

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

        ==> <element> | NIL

This function checks, whether the given element is 
included in the set.

example: >(is-element-of-set bonn *capital-cities*)
         NIL

         >(is-element-of-set bonn *cities*)
         #<CITY BONN>