theorem :: RasiowaNonClassical: p 69 (17)
  a "/\" -a <= b "\/" -b
  proof
    -(b "\/" -b) = (-b) "/\" (--b) by Th1
                .= b "/\" -b by ROBBINS3:def 6;
    then -(b "\/" -b) < -(a "/\" -a) by Def13;
    hence thesis by Def13,Th5;
  end;
