Function iter_set::union_by

pub fn union_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 union of two sets represented by sorted, deduplicated iterators, using a comparator function.

See union.