pub struct OneMore<I, P: Iterator<Item = I>> {
    parent: P,
    last: Option<I>,
    done: bool,
}

impl<I, P: Iterator<Item = I>> OneMore<I, P> {
    pub fn new(parent: P, last: I) -> Self {
        Self {
            parent,
            last: Some(last),
            done: false,
        }
    }
}

impl<I, P: Iterator<Item = I>> Iterator for OneMore<I, P> {
    type Item = I;

    fn next(&mut self) -> Option<Self::Item> {
        if self.done {
            return None;
        }
        match self.parent.next() {
            Some(item) => Some(item),
            None => {
                self.done = true;
                Some(self.last.take().unwrap())
            }
        }
    }
}