theorem  :: (2.8)
  a =-> b = Top L & b =-> c = Top L implies a =-> c = Top L
  proof
    assume a =-> b = Top L & b =-> c = Top L; then
    a "/\" b = a & b "/\" c = b by Def6; then
    a "/\" c = a by LATTICES:def 7;
    hence thesis by Def6;
  end;
