 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 Th27: :: (2.28)
  b => (a "\/" b) = Top L
  proof
    b < a "\/" b by Th7;
    hence thesis;
  end;
