pub fn iter<'a, 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::Error>where T: LoadPage, K: Storable + ?Sized, V: Storable + ?Sized, P: BTreePage<K, V>,