Function rooted_list 

fn rooted_list<'a, T: TryFromRoot<'a>>(
    root: Root<'a>,
) -> Result<Vec<T>, SexpError>
where
    T::Error: Into<SexpError>,

Each element gets its own root before conversion or the next Elt callback. Merely rooting an ALTREP list does not retain transient Elt results.