use std::collections::VecDeque;

fn permute<T: Clone>(used: &mut Vec<T>, unused: &mut VecDeque<T>, perms: &mut Vec<T>) {
    if unused.is_empty() {
        perms.extend(used.iter().cloned());
    }

    for _ in 0..unused.len() {
        used.push(unused.pop_front().unwrap());
        permute(used, unused, perms);
        unused.push_back(used.pop().unwrap());
    }
}