 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 Th13:
  (-a) "\/" b < a => b
  proof
    (a => b) "/\" ((-a) "\/" b) = (-a) "\/" b by Th12; then
    (-a) "\/" b <= a => b;
    hence thesis by Th5;
  end;
