 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 :: (2.14)
  a < b & c < d implies a "\/" c < b "\/" d & a "/\" c < b "/\" d
    by Lm2;
