pub fn iter<T, K, V, P>( txn: &'a T, db: &Db_<K, V, P>, origin: Option<(&K, Option<&V>)>) -> Result<Iter<'a, T, K, V, P>, <T as LoadPage>::Error> where T: LoadPage, K: Storable + ?Sized, V: Storable + ?Sized, P: BTreePage<K, V>,