  The minimal number of rooted subtree prune and regraft (rSPR) operations
needed to transform one phylogenetic tree into another one induces a metric on
phylogenetic trees - the rSPR-distance. The rSPR-distance between two
phylogenetic trees $T$ and $T'$ can be characterised by a maximum agreement
forest; a forest with a minimum number of components that covers both $T$ and
$T'$. The rSPR operation has recently been generalised to phylogenetic networks
with, among others, the subnetwork prune and regraft (SNPR) operation. Here, we
introduce maximum agreement graphs as an explicit representations of
differences of two phylogenetic networks, thus generalising maximum agreement
forests. We show that maximum agreement graphs induce a metric on phylogenetic
networks - the agreement distance. While this metric does not characterise the
distances induced by SNPR and other generalisations of rSPR, we prove that it
still bounds these distances with constant factors.
