code: public <T> void write(WeightedDirectedMultigraph<T> g, File f) 
              throws IOException {
          write(g, f, null);
      }
docs: Writes this {@link WeightedDirectedMultigraph} instance to the specified
  file.  Note that because the Pajek format does not support parallel
  edges, all parallel edges in one direction are flatted into a single
  directed edge.