public function addFilterRule($spec, $ruleSet)
    {
        $spec = $this->_normalizeSpec($spec);
        if (!isset($this->rules[$spec])) {
            $this->rules[$spec] = [];
        }

        if (!is_array($ruleSet)) {
            $ruleSet = [$ruleSet];
        }

        if (is_string($this->rules[$spec])) {
            $temp = $this->rules[$spec];
            $this->rules[$spec] = [];
            $this->rules[$spec][] = $temp;
        }

        foreach ($ruleSet as $rule) {
            $this->rules[$spec][] = $this->_getRule($rule);
        }

        return $this;
    }