// Merge the UnionFind<Cluster*>.
    cluster_for_node_[from].Merge(&cluster_for_node_[to]);

    return true;
  }

  string EdgeContractionFailureMsg(Cluster* from, Cluster* to,
                                   absl::string_view reason) {
    return absl::StrCat("Could not contract ", from->DebugString(*graph_),