 reserve L for Quasi-Boolean_Algebra,
         x, y, z for Element of L;
 reserve L for Nelson_Algebra,
         a, b, c, d, x, y, z for Element of L;

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;
