 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 Th17: :: (2.15)
  a "/\" (a => b) < b
  proof
    (a => b) < (a => b) by Def2;
    hence thesis by Def4;
  end;
