reserve X for set,
        A for Subset of X,
        R,S for Relation of X;

theorem Th2:
  [:X \ A,X:] \/ [:X,A:] c= [:X,X:]
  proof
    [:X,A:] c= [:X,X:] & [:X \ A,X:] c= [:X,X:] by ZFMISC_1:95;
    hence thesis by XBOOLE_1:8;
  end;
