private void findNext () {
	    while (ts.isEmpty () && !toVisit.isEmpty ()) {
		Node<T> n = toVisit.remove (toVisit.size () - 1);
		visitedNodes++;
		n.expand (mbr, converter, ts, toVisit);
	    }
	    if (ts.isEmpty ()) {
		next = null;
	    } else {
		next = ts.remove (ts.size () - 1);
		dataNodesVisited++;
	    }
	}