 reserve L for AD_Lattice;
 reserve x,y,z for Element of L;

theorem   :: Lemma 2.3. (3)
  x [= y implies x "\/" y = y "\/" x
  proof
    assume
A1: x [= y;
    ex c being Element of L st x [= c & y [= c
    proof
      take y;
      thus thesis by A1,ISum;
    end;
    hence thesis by Hehe1;
  end;
