code: void set(GvmCluster<S,K> cluster) {
  		if (cluster == this) throw new IllegalArgumentException("cannot set cluster to itself");
  		
  		m0 = cluster.m0;
  		clusters.space.setTo(m1, cluster.m1);
  		clusters.space.setTo(m2, cluster.m2);
  		var = cluster.var;
  	}
docs: Sets this cluster equal to the specified cluster
  
  @param cluster a cluster, not this or null