 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 Th34: :: (2.35)
  (-a) => (a => b) = Top L
  proof
    a "/\" -a < b by Def13; then
    (-a) < (a => b) by Def4;
    hence thesis;
  end;
