src/main/java/org/organicdesign/fp/collections/PersistentTreeMap.java
public static <K,V> PersistentTreeMap<K,V> empty(Comparator<? super K> c) {
        return new PersistentTreeMap<>(c, null, 0);
    }
public static <K,V> PersistentTreeMap<K,V> empty(Comparator<? super K> c) {
        return new PersistentTreeMap<>(c, null, 0);
    }
Returns a new empty PersistentTreeMap that will use the specified comparator.