 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 Th7:
  a < a "\/" b
  proof
    a <= a "\/" b by LATTICES:4,5;
    hence thesis by Th5;
  end;
