src/main/java/org/organicdesign/fp/collections/PersistentTreeSet.java
@Override public PersistentTreeSet<E> put(E e) {
        return (impl.containsKey(e)) ? this
                                     : new PersistentTreeSet<>(impl.assoc(e, null));
    }
@Override public PersistentTreeSet<E> put(E e) {
        return (impl.containsKey(e)) ? this
                                     : new PersistentTreeSet<>(impl.assoc(e, null));
    }
{@inheritDoc}