docs: def to_igraph(self, weighted=None):
      '''Converts this Graph object to an igraph-compatible object.
      Requires the python-igraph library.'''
      # Import here to avoid ImportErrors when igraph isn't available.
      import igraph
      ig = igraph.Graph(n=self.num_vertices(), edges=self.pairs().tolist(),
                        directed=self.is_directed())
      if weighted is not False and self.is_weighted():
        ig.es['weight'] = self.edge_weights()
      return ig