 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 Th36: :: (2.37)
  ((-!a) => a) "/\" (a => (-!a)) = Top L
  proof
A1: -!a < a by Def12;
    a < -!a by Def11;
    hence thesis by A1;
  end;
