code: void joinClusters( int mouth , int food ) {
  
  		List<Node> listMouth = clusters.get(mouth);
  		List<Node> listFood = clusters.get(food);
  
  		// put all members of food into mouth
  		for (int i = 0; i < listFood.size(); i++) {
  			listMouth.add( listFood.get(i) );
  			listFood.get(i).cluster = mouth;
  		}
  
  		// zero food members
  		listFood.clear();
  	}
docs: Moves all the members of 'food' into 'mouth'
  @param mouth The group which will not be changed.
  @param food All members of this group are put into mouth