 reserve L for Quasi-Boolean_Algebra,
         x, y, z for Element of L;

theorem Th2:
  (Top L)` = Bottom L
  proof
    thus (Top L)` = (Top L)` "\/" Bottom L
            .= (Top L)` "\/" (Bottom L)`` by ROBBINS3:def 6
            .= ((Top L) "/\" (Bottom L)`)` by Def1
            .= Bottom L by ROBBINS3:def 6;
  end;
