 reserve L for Quasi-Boolean_Algebra,
         x, y, z for Element of L;
 reserve L for Nelson_Algebra,
         a, b, c, d, x, y, z for Element of L;

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;
