[graph-tool] Merging vertices for renormalization

Hi all, I am trying to perform a renormalization on on my graph to test its self-similarity, as described in the paper by Song et al. Nature 334, 392 (2005). I have a code which assigns all vertices into different boxes, as next, I would like to merge all vertices in each box into a single, new vertex in order to get the re-normalized graph. My question: Is there better way to do it than manually re-define the whole graph? i.e. remove all vertices in a box and then re-introduce a new vertex? Is there a merge function for that maybe? Best, Hu