src/main/java/org/organicdesign/fp/collections/PersistentTreeMap.java
@Override public Comparator<? super K> comparator() {
        return (comp == Equator.Comp.DEFAULT) ? null : comp;
    }
@Override public Comparator<? super K> comparator() {
        return (comp == Equator.Comp.DEFAULT) ? null : comp;
    }
Returns the comparator used to order the keys in this map, or null if it uses
Fn2.DEFAULT_COMPARATOR (for compatibility with java.util.SortedMap).