@NotNull
  private GraphEdges oneDownStep(@NotNull GraphEdges graphEdges) {
    Set<GraphEdge> edgesInCurrentRow = graphEdges.myEdges;
    int currentRow = graphEdges.myRow;

    edgesInCurrentRow.addAll(myGraph.getAdjacentEdges(currentRow, EdgeFilter.NORMAL_DOWN));
    edgesInCurrentRow.removeAll(myGraph.getAdjacentEdges(currentRow + 1, EdgeFilter.NORMAL_UP));

    return new GraphEdges(edgesInCurrentRow, currentRow + 1);
  }
@NotNull
  private GraphEdges oneDownStep(@NotNull GraphEdges graphEdges)