  The internal model principle is a fundamental result stating a necessary and
sufficient condition for a stabilizing controller to be robustly regulating.
Its classical formulation is given in terms of coprime factorizations and the
largest invariant factor of the signal generator which sets unnecessary
restrictions for the theory and its applicability. In this article, the
internal model principle is formulated using a general factorization approach
and the generators of the fractional ideals generated by the elements of the
signal generator. The proposed results are related to the classical ones.
