/**
  * Adds one filter
  * @param  App_ListFilter_Interface          $filter
  * @param  string                            $name
  * @return App_ListFilter_CollectionAbstract
  */
 public function addFilter(App_ListFilter_Interface $filter, $name = null)
 {
     if ($filter->isValid()) {
         if (is_null($name)) {
             $this->_filters[] = $filter;
         } else {
             $this->_filters[$name] = $filter;
         }
     }
     return $this;
 }