@Nullable
  public static NormalEdge asNormalEdge(@Nullable GraphEdge edge) {
    if (isNormalEdge(edge)) {
      assert edge.getUpNodeIndex() != null && edge.getDownNodeIndex() != null;
      return NormalEdge.create(edge.getUpNodeIndex(), edge.getDownNodeIndex());
    }
    return null;
  }
@Nullable
  public static NormalEdge asNormalEdge(@Nullable GraphEdge edge)