@Joe Riel Its not a Taylor expansion around a point, but a generating function. I don't get the same results.

@Kitonum 
Since my polynom is on the form:
 (1/2 x^3+5/8 x^5+11/16 x^7+93/128 x^9) y+(-1/2 x-5/4 x^3-33/16 x^5-93/32 x^7-965/256 x^9) y^2+..
 

with the y powers collected, it only works for y^0, afterwards it gives me a fail and doesn't change it for any other power of y.