public ArrayList<Book> getSubset(int x, String xStr, int y, String yStr){
		BookData subset;
		
		//generate a subset based on the x information
		subset=getSubset(x,xStr,this);
		//refine the subset based on the y information
		subset=getSubset(y,yStr,subset);
		
		//return the list of books within this subset
		return subset.getBooks();
	}