Function flat_map_then 

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

Combination of Iterator::flat_map and then.