Procedure from library finvar.lib (see finvar_lib).
Usage:
partial_molien(M,n[,p]);
M: a 1x2 <matrix>, n: an <int> indicating number of terms in the
expansion, p: an optional <poly>
Assume:
M is the return value of molien or the second return value of
reynolds_molien, p ought to be the second return value of a previous
run of partial_molien and avoids recalculating known terms
Return:
n terms (type <poly>) of the partial expansion of the Molien series
(first n if there is no third parameter given, otherwise the next n
terms depending on a previous calculation) and an intermediate result
(type <poly>) of the calculation to be used as third parameter in a
next run of partial_molien