/* package */static Tracker getBackedTracker (Collection<String> announces) {
        List<Tracker> trackers = new ArrayList<Tracker>();

        for (String str : announces) {
            Tracker tracker = getSingleTracker(str);
            trackers.add(tracker);

        }

        if (trackers.isEmpty()) {
            return null;
        }

        if (trackers.size() == 1) {
            return trackers.get(0);
        }

        return new BackedTracker(trackers);
    }