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

theorem LemXX:   :: Lemma 2.3. (9)
  (x "/\" y) "\/" y = y
  proof
    (x "/\" y) "\/" y = (x "/\" y) "\/" (y "/\" y) by IMeet
                     .= (x "\/" y) "/\" y by DefD
                     .= y by DefA1;
    hence thesis;
  end;
