### generating series

Hi. I want to create a function which will pick out the coefficients of a generating series. In particular, I have a generating series $\sum_{k=0}^\infty a_kt^k$ defined by an infinite product $\prod_{k=1}^\infty P_k(t)$. How do I just pick out the coefficients a_k?

I'm using Sage 4.5.3 on Mac OS X 10.6.4.

Thanks for the help!!!!!!!!!