static <T extends ExternalizableGraphElement> Externalizer<T> forGraphElement(DataReader<? extends T> reader) {
    return new Externalizer<>() {
      @Override
      public T load(GraphDataInput in) throws IOException {
        return reader.load(in);
      }

      @Override
      public void save(GraphDataOutput out, T value) throws IOException {
        value.write(out);
      }
    };
  }
static <T extends ExternalizableGraphElement> Externalizer<T> forGraphElement(DataReader<? extends T> reader)