@SuppressWarnings("unchecked")
    public static <K extends Comparable<K>, V> PersistentTreeMap<K,V> empty() {
        return (PersistentTreeMap<K,V>) EMPTY;
    }
@ SuppressWarnings ( "unchecked" ) public static < K extends Comparable < K > , V > PersistentTreeMap < K , V > empty ( ) { return ( PersistentTreeMap < K , V > ) EMPTY ; }
Be extremely careful with this because it uses the default comparator, which only works for
items that implement Comparable (have a "natural ordering").  An attempt to use it with other
items will blow up at runtime.  Either a withComparator() method will be added, or this will
be removed.