use std::collections::VecDeque;

struct A {
    a: u64,
    b: VecDeque<(usize, u64)>,
}

struct B(u64, u64);

fn insert(id: usize, dst: &mut VecDeque<A>, src: &[B]) {

    let (mut insert_iter, x) = if let Some(v) = dst.front() {
        (src.iter(), v.a)
    } else {
        let mut insert_iter = src.iter();
        if let Some(a) = insert_iter.next() {
            (insert_iter, a.0)
        } else {
            return;
        }
    };

    for l in insert_iter {
        // do stuff
    }
}

fn main() {
    println!("Hello, world!");
}