fn main() {
    let mut a = 0;
    let mut b = 1;

    let fibs = move || {
        let next = a + b;
        a = b;
        b = next;
        Some(next)
    };

    let fibs = || std::iter::from_fn(fibs);

    for fib in fibs().take(10) {
        println!("{}", fib)
    }

    println!("{:?}", fibs().take(20).collect::<Vec<_>>())
}