src/main/java/org/organicdesign/fp/collections/PersistentTreeMap.java
@Override public ImSortedSet<Entry<K,V>> entrySet() {
        // This is the pretty way to do it.
        return this.fold(PersistentTreeSet.ofComp(new KeyComparator<>(comp)),
                         PersistentTreeSet::put);
    }
@Override public ImSortedSet<Entry<K,V>> entrySet() {
        // This is the pretty way to do it.
        return this.fold(PersistentTreeSet.ofComp(new KeyComparator<>(comp)),
                         PersistentTreeSet::put);
    }
Returns a view of the mappings contained in this map.  The set should actually contain
UnmodMap.UnEntry items, but that return signature is illegal in Java, so you'll just have to
remember.