If you have a node that was promoted to a non-root
graph because it was connected with an edge (i.e. a
sub-graph's acceptsConnectedNode() returns true for it)
and you subsequently remove that edge, the node is left
in the non-root graph, even if there is no reason for
it to be there anymore. It should be removed and
placed in the root graph (unless it is in another graph
already).
