fn main() {
let xs=[1_i64,4,9,16];
fn each_pair<T: Copy>(xs: &[T], mut f: impl FnMut(T, T)) {
    for pair in xs.windows(2) { f(pair[0], pair[1]); }
}
let mut gaps: Vec<i64> = Vec::new();
each_pair(&xs, |a, b| gaps.push(b - a));

assert_eq!(gaps,[3,5,7]);each_pair(&[] as &[i64],|_,_|panic!());each_pair(&[1_i64],|_,_|panic!());
}