* This class provides a skeletal implementation of {@link BaseGraph}.
 *
 * <p>The methods implemented in this class should not be overridden unless the subclass admits a
 * more efficient implementation.
 *
 * @author James Sexton
 * @param <N> Node parameter type
 */
@ElementTypesAreNonnullByDefault
abstract class AbstractBaseGraph<N> implements BaseGraph<N> {

  /**