`diff'
------

diff(RAT[,VARN]*)
diff(RAT,VARLIST)
     :: RAT  VARN 뤤 VARLIST ѿǽ缡ʬ.

RETURN
     
RAT
     ͭ (ȡޤǤ褤)
VARN
     긵
VARLIST
     긵Υꥹ

   * Ϳ줿ȡ VARN 뤤 VARLIST ѿǽ缡ʬ
     .
   * ¦긵, ʬƤ. Ĥޤ, diff(RAT,x,y) ,
     diff(diff(RAT,x),y) ƱǤ.

     [0] diff((x+2*y)^2,x);  
     2*x+4*y
     [1] diff((x+2*y)^2,x,y);
     4
     [2] diff(x/sin(log(x)+1),x);
     (sin(log(x)+1)-cos(log(x)+1))/(sin(log(x)+1)^2)
     [3] diff(sin(x),[x,x,x,x]);
     sin(x)

