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

theorem    :: Lemma 2.3. (9)
  x "\/" (y "/\" x) = x
  proof
    x "\/" (y "/\" x) = (x "/\" x) "\/" (y "/\" x) by IMeet
      .= (x "\/" y) "/\" x by DefD
      .= x by DefA2;
    hence thesis;
  end;
