 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 Th30: :: (2.31)
  (a "/\" b) => b = Top L
  proof
    a "/\" b < b by Th6;
    hence thesis;
  end;
