pub fn recursive(pset: &mut PrimeSet, product: u32) {
    for i in 0.. {
        let p = pset.iter().nth(i).unwrap();
        let a = p as u32 * product;
        if a > 40 {
            return;
        } else {
            recursive(pset, a);
        }
    }
}

// stubs
pub struct PrimeSet;
pub struct PrimeSetIter<'a>(std::marker::PhantomData<&'a mut ()>);
impl<'a> Iterator for PrimeSetIter<'a> {
    type Item = u64;
    fn next(&mut self) -> Option<u64> {
        Some(2)
    }
}

impl PrimeSet {
    pub fn iter<'a>(&'a mut self) -> PrimeSetIter<'a> {
        PrimeSetIter(std::marker::PhantomData)
    }
}