combinations.p6

use MONKEY_TYPING;

sub combinations(Int $n, Int $k) {
    return [] if $k == 0;
    return () if $k > $n;
    gather {
        take [0, (1..^$n)[@$_]] for combinations($n-1, $k-1);
        take [(1..^$n)[@$_]]    for combinations($n-1, $k  );
    }
}

augment class List {
    method combinations(Int $n) {
        gather take self[@$_] for combinations self.elems, $n
    }
}

say  (^6).list.combinations(3).perl;