pub struct UnitVec {
    vec: Vec<Unit>,
    vacants: BTreeSet<usize>
}

impl UnitVec {
    pub fn iter_mut<'a>(&'a mut self) -> UnitIterMut<'a> {
        UnitIterMut {
            vec: &mut self.vec,
            vacants: self.vacants.iter(),
            count: 0,
        }
    }
}

struct UnitIterMut<'a> {
    vec: &'a mut Vec<Unit>,
    vacants: btree_set::Iter<'a, usize>,
    count: usize,
}

impl<'a> Iterator for UnitIterMut<'a> {
    type Item = &'a mut Unit;
    fn next(&mut self) -> Option<Self::Item> {
        if self.count >= self.vec.len() {
            return None;
        } else if Some(&&self.count) == self.vacants.peekable().peek() {
            let _ = self.vacants.next();
            return self.next();
        } else {
            self.count += 1;
            return Some(self.vec[self.count - 1]);
        }
    }
}