object SetDB
    components: Elem*;     (* Note the minimal representation *)

    operations:
        Insert (SetDB, Elem) -> (SetDB),  (* Constructor operation *)
        Delete (SetDB, Elem) -> (SetDB),  (* Destructor operation *)
        Find (SetDB, Elem) -> (boolean),  (* Selector operation *)
        EmptyDB () -> (SetDB);            (* Initializer operation *)

    equations:
        var s: SetDB;
        var e, e': Elem;
        Find(EmptyDB(), e) == false;
        Find(Insert(s, e), e') ==
            if e=e' then true else Find(s, e');
        Delete(EmptyDB(), e) == EmptyDB();
        Delete(Insert(s, e), e') ==
            if e=e'
            then Delete(s, e')
            else Insert(Delete(s, e'), e);

end SetDB;