@NotNull
  public Set<GraphEdge> getEdgesInRow(int rowIndex) {
    GraphEdges neighborU = getNeighborU(rowIndex);
    while (neighborU.myRow < rowIndex) {
      neighborU = oneDownStep(neighborU);
    }

    GraphEdges neighborD = getNeighborD(rowIndex);
    while (neighborD.myRow > rowIndex) {
      neighborD = oneUpStep(neighborD);
    }

    Set<GraphEdge> result = neighborU.myEdges;
    result.addAll(neighborD.myEdges);
    return result;
  }
@NotNull
  public Set<GraphEdge> getEdgesInRow(int rowIndex)