code: def write_combined_transit_stop_to_stop_network(gtfs, output_path, fmt=None):
      
      if fmt is None:
          fmt = "edg"
      multi_di_graph = combined_stop_to_stop_transit_network(gtfs)
      _write_stop_to_stop_network_edges(multi_di_graph, output_path, fmt=fmt)
docs: Parameters
      ----------
      gtfs : gtfspy.GTFS
      output_path : str
      fmt: None, optional
          defaulting to "edg" and writing results as ".edg" files
           If "csv" csv files are produced instead