igraph data structure versionsSource:
igraph's internal data representation changes sometimes between versions. This means that it is not possible to use igraph objects that were created (and possibly saved to a file) with an older igraph version.
The input graph. If it is missing, then the version number of the current data format is returned.
graph_version() queries the current data format,
or the data format of a possibly older igraph graph.
upgrade_graph() can convert an older data format
to the current one.
upgrade_graph to convert the data format of a graph.