  We study the closure operators of polymatroids from a lattice theoretic point
of view. We show that polymatroid closure operators relate to lattices enriched
with a generating set in the same way that matroids relate to geometric
lattices. Through this relation we define a notion of minors for lattices
enriched with a generating set. For the lattice of flats of a graphic matroid,
the minors of the lattice are shown to correspond to simple minors of the graph
when the vertices are labeled and the edges unlabeled. This correspondence is
generalized to all polymatroids.
