 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 Th16bis:
  x < y implies a => x < a => y
  proof
    assume x < y; then
    a "/\" (a => x) < y by Th16;
    hence thesis by Def4;
  end;
