*/
@ElementTypesAreNonnullByDefault
final class UndirectedNetworkConnections<N, E> extends AbstractUndirectedNetworkConnections<N, E> {

  UndirectedNetworkConnections(Map<E, N> incidentEdgeMap) {
    super(incidentEdgeMap);
  }

  static <N, E> UndirectedNetworkConnections<N, E> of() {
    return new UndirectedNetworkConnections<>(HashBiMap.<E, N>create(EXPECTED_DEGREE));
  }

            ? DirectedMultiNetworkConnections.<N, E>of()
            : DirectedNetworkConnections.<N, E>of()
        : allowsParallelEdges()
            ? UndirectedMultiNetworkConnections.<N, E>of()
            : UndirectedNetworkConnections.<N, E>of();
  }

          Maps.asMap(network.incidentEdges(node), adjacentNodeFn(network, node));
      return network.allowsParallelEdges()
          ? UndirectedMultiNetworkConnections.ofImmutable(incidentEdgeMap)
          : UndirectedNetworkConnections.ofImmutable(incidentEdgeMap);
    }
  }

  private static <N, E> Function<E, N> sourceNodeFn(Network<N, E> network) {
    return (E edge) -> network.incidentNodes(edge).source();
  }