Function iter_set::cmp_by_key

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

Compare two sets represented by sorted, deduplicated iterators, using a key extraction function.

See cmp.