src/main/java/org/organicdesign/fp/collections/PersistentTreeMap.java
@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.