optimization done in ExtFlat, which is to keep a count of the
number of different node names assigned to the node so that when
merging, the one with fewer nodes can be updated to match the one
with more nodes. Note: This change is made on the assumption
that the names for node1 and node2 are equally preferred.
Supposedly the first name in the node list is canonical, so if
node1 is preferred in any case, it may be necessary to move
the first item of the second list to the beginning (a minor code
change).