Function missing_keys 

pub fn missing_keys<'a, K: IntoIterator<Item = &'a str>>(
    layout: &Layout,
    wanted: impl IntoIterator<Item = (usize, K)>,
) -> Result<Vec<Vec<Arc<str>>>, Error>

For each axis of layout, the keys that wanted names on it and it lacks, each once, in order. wanted pairs an axis with the keys that one selector names on it. A write adds these keys before it plans (extended).