theorem ThA4:   :: Lemma 3.4. (A4)
  x "\/" (x "/\" y) = x
  proof
    thus x "\/" (x "/\" y) = (x "\/" x) "/\" (x "\/" y) by DefLDS
       .= x "/\" (x "\/" y) by ISum
       .= x by LATTICES:def 9;
  end;
