theorem Th8:
  -(a "/\" b) = (-a) "\/" (-b)
  proof
    thus -(a "/\" b) = -((-(-a)) "/\" b) by ROBBINS3:def 6
               .= -((-(-a)) "/\" (-(-b))) by ROBBINS3:def 6
               .= -(-((-a) "\/"(- b))) by Th1
               .= ((-a) "\/" (-b)) by ROBBINS3:def 6;
  end;
