 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 Th32: :: (2.33)
  (a => !b) => (b => !a) = Top L
  proof
    a => !b = b => !a by Th20;
    hence thesis by Th16ter;
  end;
