void Merge(Cluster* other);

    // If this is a trivial cluster containing only one node then return the ID
    // of that node.  May not be called otherwise.
    int GetIdOfOnlyNode() const {
      DCHECK_EQ(cluster_size(), 1);
      return cycles_graph_node_id();
    }

    // The number of TF nodes in this cluster.
    int cluster_size() const { return cluster_size_; }