struct CustomIter {
mut:
    idx int
}

fn new_custom_iter() CustomIter {
    return CustomIter{
        idx: 0
    }
}

fn (mut a CustomIter) next() ?int {
    if a.idx == 4 {
        return none
    } else {
        a.idx++
        return a.idx * 2
    }
}

fn main() {
    for x in new_custom_iter() {
        println('a.${x}')
    }
    for ix, val in new_custom_iter() {
        println('b.${ix}=${val}')
    }
    println('end')
}