`vars'
------

vars(OBJ)
     :: OBJ ˴ޤޤѿΥꥹ.

RETURN
     ꥹ
OBJ
     Ǥ

   * Ϳ줿˴ޤޤѿΥꥹȤ֤.
   * ѿι⤤Τ¤٤.

     [0] vars(x^2+y^2+a^2);
     [x,y,a]
     [1] vars(3/abc+2*xy/efg);
     [abc,xy,efg]
     [2] vars([x,y,z]);
     [x,y,z]


     `var', `uc', `ord'

