 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 Th19: :: (2.18)
  a < ((a => b) => b)
  proof
    a "/\" (a => b) < b by Th17;
    hence thesis by Def4;
  end;
