 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 Th19bis: :: (2.19)
  a < b => (a "/\" b)
  proof
    b "/\" a <= a "/\" b; then
    b "/\" a < a "/\" b by Th5;
    hence thesis by Def4;
  end;
