public function set($post_id, string $key, array $values)
    {
        $post_id = (int) ($post_id ?? get_the_ID());

        if (!$post_id || !$key) {
            return; // Not possible.
        }
        $this->unset($post_id, $key);

        $key = $this->key($key);
        foreach ($values as $_value) {
            add_post_meta($post_id, $key, $_value);
        } // unset($_value); // Housekeeping.
    }