public function addCategory(array $category)
    {
        if (empty($category['term'])) {
            /**
             * @see Zend_Feed_Builder_Exception
             */
            require_once 'Zend/Feed/Builder/Exception.php';
            throw new Zend_Feed_Builder_Exception("you have to define the name of the category");
        }

        if (!$this->offsetExists('category')) {
            $categories = array($category);
        } else {
            $categories = $this->offsetGet('category');
            $categories[] = $category;
        }
        $this->offsetSet('category', $categories);
        return $this;
    }