pub fn comb<T>(slice: &[T], k: usize) -> Vec<Vec<T>>
where
T: Copy,
{
if k == 1 {
return slice.iter().map(|x| vec![*x]).collect::<Vec<Vec<T>>>();
}
if k == slice.len() {
return vec![slice.to_vec()];
}
let mut result = comb(&slice[1..], k - 1)
.into_iter()
.map(|x| [&slice[..1], x.as_slice()].concat())
.collect::<Vec<Vec<T>>>();
result.extend(comb(&slice[1..], k));
return result;
}