Example:
ring R= 0,(x,y,z,u),dp; ideal i=-3zu+y2-2x+2, -3x2u-4yz-6xz+2y2+3xy, -3z2u-xu+y2z+y; print(char_series(i)); ==> _[1,1],3x2z-y2+2yz,3x2u-3xy-2y2+2yu, ==> x, -y+2z, -2y2+3yu-4