src/main/java/org/organicdesign/fp/collections/PersistentTreeSet.java
public static <T> PersistentTreeSet<T> ofComp(Comparator<? super T> comp) {
        return new PersistentTreeSet<>(PersistentTreeMap.empty(comp));
    }
public static <T> PersistentTreeSet<T> ofComp(Comparator<? super T> comp) {
        return new PersistentTreeSet<>(PersistentTreeMap.empty(comp));
    }
Returns a new PersistentTreeSet of the given comparator.  Always use this instead of starting
with empty() because there is no way to assign a comparator to an existing set.