 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 ThA5:   :: Lemma 3.4. (A5)
  x "\/" (y "/\" x) = x
  proof
    thus x "\/" (y "/\" x) = (x "\/" y) "/\" (x "\/" x) by DefLDS
       .= (x "\/" y) "/\" x by ISum
       .= x by DefA2;
  end;
