@SuppressWarnings("unchecked")
    public V popLowestKeyValue() {
        if (firstNonEmpty == null) {
            return null;
        }
        size--;
        final SortedBin minBin = firstNonEmpty.getMinBin();
        return (V) minBin.dropLowest();
    }
@return the lowest key value, or null if none
main/src/main/java/cgeo/geocaching/brouter/util/SortedHeap.java