 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 Th18: :: (2.17)
  a => (b => c) = b => (a => c)
  proof
    a => (b => c) = (a "/\" b) => c by Lm3
      .= b => (a => c) by Lm3;
    hence thesis;
  end;
