fn diff_field_sorted_iter<'a, K, V, IF, IT>(
    from: IF,
    to: IT,
    diffs: &mut Vec<StateFieldDiff<K, V>>
)where
    K: Ord + Clone + 'a,
    V: PartialEq + Clone + 'a,
    IF: IntoIterator<Item = (&'a K, &'a V)>,
    IT: IntoIterator<Item = (&'a K, &'a V)>,