code: void add(GvmCluster<S,K> cluster) {
  		if (cluster == this) throw new IllegalArgumentException();
  		if (cluster.count == 0) return; //nothing to do
  		
  		if (count == 0) {
  			set(cluster);
  		} else {
  			count += cluster.count;
  			//TODO accelerate add
  			m0 += cluster.m0;
              clusters.space.add(m1, cluster.m1);
              clusters.space.add(m2, cluster.m2);
  			update();
  		}
  	}
docs: Adds the specified cluster to this cluster.
  
  @param cluster the cluster to be added