public function addConditionalRules($attribute, $rules = [])
    {
        foreach ((array) $attribute as $key) {
            $current = isset($this->conditional[$key]) ? $this->conditional[$key] : [];
            $merge = head($this->validator->explodeRules((array) $rules));
            $this->conditional[$key] = array_merge($current, $merge);
        }
    }