Function flat_map_then_with 

pub fn flat_map_then_with<'a, T: Clone + 'a, U: 'a, V: 'a, E: 'a>(
    l: impl Iterator<Item = Result<U, E>> + 'a,
    x: T,
    r: impl Fn(U, T) -> Results<'a, V, E> + 'a,
) -> Results<'a, V, E>

Combination of flat_map_with and then.