pub fn compute_position<'store, HAST, It>(
    root: HAST::IdN,
    offsets: &mut It,
    stores: &'store HAST
) -> (Position, HAST::IdN)where
    It::Item: Clone,
    HAST::IdN: Clone,
    HAST: HyperAST<'store>,
    HAST::T: WithSerialization + WithChildren,
    It: Iterator<Item = HAST::Idx>,