protected final void validateEndpoints(EndpointPair<?> endpoints) {
    checkNotNull(endpoints);
    checkArgument(isOrderingCompatible(endpoints), ENDPOINTS_MISMATCH);
  }

  /**
   * Returns {@code true} iff {@code endpoints}' ordering is compatible with the directionality of
   * this graph.
   */
  protected final boolean isOrderingCompatible(EndpointPair<?> endpoints) {
    return endpoints.isOrdered() == this.isDirected();