Function yield_ Copy item path

6fn sum(a: u32) -> u32 {
7    let mut sum = a;
8    let mut recv: u32;
9    while sum < 200 {
10        // println!("sum={} ", sum);
11        recv = yield_(sum).unwrap();
12        // println!("recv={}", recv);
13        sum += recv;
14    }
15    sum
16}