`nm', `dn'
----------

nm(RAT)
     :: RAT ʬ.
dn(RAT)
     :: RAT ʬ.

RETURN
     ޤ¿༰
RAT
     ͭޤͭ

   * Ϳ줿ͭޤͭʬҵڤʬ֤.
   * ͭξ, ʬϾ, ʬҤ.
   * ͭξ, ñʬ, ʬҤФǤ. ͭФ
     , ʬϼưŪˤϹԤʤ. `red()'Ū˸ƤӽФɬפ
     .

     [2] [nm(-43/8),dn(-43/8)];
     [-43,8]
     [3] dn((x*z)/(x*y));
     y*x
     [3] dn(red((x*z)/(x*y)));
     y


     `red'

