 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 Th21: :: (2.21)
  (a => Top L) = Top L
  proof
    a <= Top L by LATTICES:4,19; then
    a < Top L by Th5;
    hence thesis;
  end;
