Function classify_by 

pub fn classify_by<T, L, R, F>(
    a: L,
    b: R,
    cmp: F,
) -> ClassifyBy<L::IntoIter, R::IntoIter, F> 
where
    L: IntoIterator<Item = T>,
    R: IntoIterator<Item = T>,
    F: FnMut(&mut T, &mut T) -> Ordering,

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

See classify().