Function iter_set::difference_by

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

Take the difference of two sets represented by sorted, deduplicated iterators, using a comparator function.

See difference().