// We're relying on callers to call this method only with an edge that's in the graph.
    return requireNonNull(incidentEdgeMap.get(edge));
  }

  @Override
  @CheckForNull
  public N removeInEdge(E edge, boolean isSelfLoop) {
    if (!isSelfLoop) {
      return removeOutEdge(edge);
    }
    return null;
  }

  @Override
  public N removeOutEdge(E edge) {