protected function _addFilter($facetName, $value)
    {
        if (isset($this->filters[$facetName])) {
            $currentValue = $this->filters[$facetName];
            if (!is_array($currentValue)) {
                $currentValue = array($currentValue);
            }
            $currentValue[] = $value;
            $value = $currentValue;
        }
        $this->filters[$facetName] = $value;
    }