code: void setClusterUserIndex(int cluster, int index, int value) {
  		int i = getClusterIndex_(cluster);
  		AttributeStreamOfInt32 stream = m_clusterIndices.get(index);
  		if (stream.size() <= i)
  			stream.resize(m_clusterData.size(), -1);
  
  		stream.write(i, value);
  	}
docs: Sets a user index value for the cluster.