* @param <N> Node parameter type
 * @param <E> Edge parameter type
 */
abstract class AbstractUndirectedNetworkConnections<N, E> implements NetworkConnections<N, E> {
  /** Keys are edges incident to the origin node, values are the node at the other end. */
  final Map<E, N> incidentEdgeMap;

  AbstractUndirectedNetworkConnections(Map<E, N> incidentEdgeMap) {
    this.incidentEdgeMap = checkNotNull(incidentEdgeMap);
  }

 *
 * @author James Sexton
 * @param <N> Node parameter type
 * @param <E> Edge parameter type
 */
final class UndirectedNetworkConnections<N, E> extends AbstractUndirectedNetworkConnections<N, E> {

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

  static <N, E> UndirectedNetworkConnections<N, E> of() {