# otherwise..
    my $include_iter = iter_choose_n(@rest, $n-1);
    my $exclude_iter;
    return sub {
        if ($include_iter and my $set = $include_iter->()) {
            return [$first, @$set];
        }
        else {
            if ($include_iter) {
                undef $include_iter;
                $exclude_iter = iter_choose_n(@rest, $n);
            }
            return $exclude_iter->();
        }
    }
}