converting a multivariate series to a polynomial

[Max Alekseyev <maxale@gmail.com>]

While it is easy to convert univariate t_SER to t_POL by calling Pol(), I'm puzzled on what would be the easiest way to convert a multivariate series like

((((x5 + O(x5^10))*x4 + O(x4^10))*x3 + O(x3^10))*x2 + O(x2^10))*x1 + O(x1^10)

to a polynomial, which is x5*x4*x3*x2*x1 in this example. 

Thanks,
Max