Function classify_by_key 

pub fn classify_by_key<T, L, R, K, F>(
    a: L,
    b: R,
    key: F,
) -> ClassifyByKey<L::IntoIter, R::IntoIter, F> 
where
    L: IntoIterator<Item = T>,
    R: IntoIterator<Item = T>,
    K: Ord,
    F: FnMut(&T) -> K,

Interleave two sorted, deduplicated iterators in sorted order and classify each element according to its source, using a key extraction function.

See classify().