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

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