% problem-set/topology/point.set/lemma21.clauses % created : 08/08/88 % revised : 08/17/89 % description : % % lemma 21: closure(xa) = interior(xa) union boundary(xa). % representation : % % See axioms.decl for a full explanation. % denial: -eq_s(closure(A,X,T),join(interior(A,X,T),boundary(A,X,T))).