Interface for listeners attached to partitioned graphs.
two different listener interfaces, providing different types of events: while
provides only information about the graph as a whole
(nodes/edges added or removed),
provides information about the partitioned structure of the graph (partitions/boundary
edges added, removed or changed).
Modifying the structure of a partitioned graph can thus result in two events
being generated: one for modifications to the graph, as a normal graph, and one
for modifications to the graph, as a partitioned graph.