 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 Th22: :: (2.22)
  ((Bottom L) => a) = Top L
  proof
    Bottom L <= a; then
    Bottom L < a by Th5;
    hence thesis;
  end;
