`coef'
------

coef(POLY,DEG[,VAR])
     :: POLY  VAR (άϼѿ) ˴ؤ DEG η.

RETURN
     ¿༰
POLY
     ¿༰
VAR
     긵
DEG
     

   * POLY  VAR ˴ؤ DEG ηϤ.
   * VAR , άȼѿ var(POLY) Ȥߤʤ.
   * VAR ѿǤʤ, VAR ѿξӤƸΨ.

     [0] A = (x+y+z)^3;
     x^3+(3*y+3*z)*x^2+(3*y^2+6*z*y+3*z^2)*x+y^3+3*z*y^2+3*z^2*y+z^3
     [1] coef(A,1,y);
     3*x^2+6*z*x+3*z^2
     [2] coef(A,0);
     y^3+3*z*y^2+3*z^2*y+z^3


     `var', `deg'

