theorem Th4:
  a "/\" b = Top L iff a = Top L & b = Top L
  proof
    hereby assume
A1:   a "/\" b = Top L; then
      Top L [= a by LATTICES:6;
      hence a = Top L;
      Top L [= b by A1,LATTICES:6;
      hence b = Top L;
    end;
    thus thesis;
  end;
