@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));
    }
@ Override public PersistentTreeSet < E > put ( E e ) { return ( impl . containsKey ( e ) ) ? this : new PersistentTreeSet <> ( impl . assoc ( e , null ) ) ; }
{@inheritDoc}
{