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

theorem ISum:   :: Lemma 3.4. (I \/)
  x "\/" x = x
  proof
    thus x "\/" x = ((x "\/" x) "/\" x) "\/" x by DefA2
      .= x by LATTICES:def 8;
  end;
