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

theorem Ze:   :: Lemma 2.3. (9)
  x "/\" (x "\/" y) = x
  proof
    x "/\" (x "\/" y) = (x "/\" x) "\/" (x "/\" y) by LATTICES:def 11
      .= x "\/" (x "/\" y) by IMeet
      .= x by ROBBINS3:def 3;
    hence thesis;
  end;
