`coef'
------

coef(POLY,DEG[,VAR])
     :: POLY $@$N(J VAR ($@>JN,;~$O<gJQ?t(J) $@$K4X$9$k(J DEG $@<!$N78?t(J.

RETURN
     $@B?9`<0(J
POLY
     $@B?9`<0(J
VAR
     $@ITDj85(J
DEG
     $@<+A3?t(J

   * POLY $@$N(J VAR $@$K4X$9$k(J DEG $@<!$N78?t$r=PNO$9$k(J.
   * VAR $@$O(J, $@>JN,$9$k$H<gJQ?t(J var(POLY) $@$@$H$_$J$5$l$k(J.
   * VAR $@$,<gJQ?t$G$J$$;~(J, VAR $@$,<gJQ?t$N>l9g$KHf3S$7$F8zN($,Mn$A$k(J.

     [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

$@;2>H(J
     `var', `deg'

