fn f(a : &mut char, b : &mut char) {
    println!("a: {:?} b: {:?}", a, b);
    if *a == 'b' && *b == 'c' {
        *a = 'x';
    }
}

fn main() {
    let mut vec = vec!['a', 'b', 'c', 'd'];
    for i in 1..vec.len() {
        let (hd, tl) = vec.split_at_mut(i);
        let mut x = hd.last_mut().unwrap();
        for mut y in tl.iter_mut() {
            f(&mut x, &mut y);
        }
    }
    println!("vec: {:?}", vec);
}