code: public function addAttribute(/*# string */ $attrName, $attrValue)
      {
          if (!is_array($this->attributes[$attrName])) {
              $this->attributes[$attrName] = (array) $this->attributes[$attrName];
          }
          $this->attributes[$attrName][] = $attrValue;
          return $this;
      }
docs: {@inheritDoc}