3.2. igraph_write_graph_graphml — Writes the graph to a file in GraphML format

int igraph_write_graph_graphml(const igraph_t *graph, FILE *outstream);

GraphML is an XML-based file format for representing various types of graphs. See the GraphML Primer (http://graphml.graphdrawing.org/primer/graphml-primer.html) for detailed format description.

Arguments:

`graph`:	The graph to write.
`outstream`:	The stream object to write to, it should be writable.

Returns:

Error code: IGRAPH_EFILE if there is an error writing the file.

Time complexity: O(|V|+|E|) otherwise. All file operations are expected to have time complexity O(1).