fn f<'a>(mut a : &'a mut char, mut b : &'a mut char) {
    for x in vec![&mut a, &mut b] {
        /*if x == 'a' {
            *x = 'x';
        }*/
        println!("x: {:?}", x);
    }
    println!("a: {:?} b: {:?}", a, b);
}

fn main() {
    let mut a = 'a';
    let mut b = 'b';
    f(&mut a, &mut b);
}