struct Fibonacci {t: (u64, u64)}
impl Iterator for Fibonacci {
    type Item = u64;
    fn next(&mut self) -> Option<Self::Item> {
        let current = Some(self.t.0);
        self.t = (self.t.1, self.t.0 + self.t.1);
        current
    }
}
fn main() {
    let fib = |a, b| Fibonacci {t: (a, b)};
    let f = |a, b, c| fib(a, b).take_while(|&x| x <= c).position(|x| x == c);
    for c in 1..6 {println!("c: {}, pos: {:?}", c, f(1, 1, c))}
}
 

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