 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 Th3:
  a <= b & b <= a iff a = b
  proof
    thus a <= b & b <= a implies a = b
    proof
      assume a <= b & b <= a; then
      a [= b & b [= a by LATTICES:4;
      hence thesis by LATTICES:8;
    end;
    thus thesis;
  end;
