public function add_filter($tag, $function_to_add, $priority = 10, $accepted_args = 1)
    {
        $idx = self::_filter_build_unique_id($tag, $function_to_add, $priority);
        $this->filter[ $tag ][ $priority ][ $idx ] = array(
            'function'      => $function_to_add,
            'accepted_args' => $accepted_args,
        );
        unset($this->merged_filters[ $tag ]);

        return true;
    }