pub trait ElementExt {
fn scalar<K, T>(&self, key: K) -> Result<T, PlyError>
where
K: AsRef<str>,
T: NumCast;
fn list<K, T, I>(&self, key: K) -> Result<I, PlyError>
where
K: AsRef<str>,
T: NumCast,
I: FromIterator<T>;
}