this.nodeConnections =
        (nodeConnections instanceof TreeMap)
            ? new MapRetrievalCache<N, NetworkConnections<N, E>>(nodeConnections)
            : new MapIteratorCache<N, NetworkConnections<N, E>>(nodeConnections);
    this.edgeToReferenceNode = new MapIteratorCache<>(edgeToReferenceNode);
  }

  @Override
  public Set<N> nodes() {

  }

  private static <N, E> Map<N, NetworkConnections<N, E>> getNodeConnections(Network<N, E> network) {
    // ImmutableMap.Builder maintains the order of the elements as inserted, so the map will have
    // whatever ordering the network's nodes do, so ImmutableSortedMap is unnecessary even if the
    // input nodes are sorted.
    ImmutableMap.Builder<N, NetworkConnections<N, E>> nodeConnections = ImmutableMap.builder();