 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 :: RasiowaNonClassical: p 69 (18)
  a <= b =-> (a "/\" b)
  proof
A1: a < b => (b "/\" a) & a < (-(b "/\" a)) => -b by Lm5,Th19bis;
    -( b =-> (b "/\" a)) < -a by Lm6;
    hence thesis by A1,Th5,Def8;
  end;
