code: def layerset2str(discoursegraph):
      
      for node_id in discoursegraph:
          discoursegraph.node[node_id]['layers'] = \
              str(discoursegraph.node[node_id]['layers'])
      for (from_id, to_id) in discoursegraph.edges_iter():
          # there might be multiple edges between 2 nodes
          edge_dict = discoursegraph.edge[from_id][to_id]
          for edge_id in edge_dict:
              edge_dict[edge_id]['layers'] = \
                  str(edge_dict[edge_id]['layers'])
docs: typecasts all `layers` from sets of strings into a single string to make
      the graph exportable (e.g. into the `gexf` and `graphml` formats).
  
      Parameters
      ----------
      discoursegraph : DiscourseDocumentGraph