  Let M and N be two r x r matrices over a discrete valuation ring of
characteristic zero. The orders (with respect to a uniformizing parameter) of
the invariant factors of M form a partition of non-negative integers, called
the invariant partition of M. Let the invariant partition of M be mu, of N be
nu, and of the product MN be lambda. In this paper we construct a
Littlewood-Richardson filling of the skew shape lambda/mu with content nu, and
show that this filling is an invariant of the orbit of the pair (M,N) with
respect to a natural group action on the pair. We relate the algebraic
combinatorics of Littlewood-Richardson fillings to a special semicanonical
matrix in the orbit of (M,N), from which the Littlewood-Richardson filling, and
other combinatorial invariants may be obtained.
