 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 Th24: :: (2.25)
  a => (b => a) = Top L
  proof
    b "/\" a <= a by LATTICES:4,6; then
    b "/\" a < a by Th5; then
    a < b => a by Def4;
    hence thesis;
  end;
