 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 Th33: :: (2.34)
  (!(a => a)) => b = Top L
  proof
A1: a => a = Top L by Th16ter;
A2: !(Top L) = ((Top L) => - (Top L)) by Def14;
    (!a => a) => b = ((Top L) => (Bottom L)) => b by Th2,A2,A1
    .= (Bottom L) => b by Th23
    .= Top L by Th22;
    hence thesis;
  end;
