public function generateChildDef(&$def, $module)
    {
        if (!empty($def->child)) { // already done!
            return;
        }
        $content_model = $def->content_model;
        if (is_string($content_model)) {
            // Assume that $this->keys is alphanumeric
            $def->content_model = preg_replace_callback(
                '/\b(' . implode('|', $this->keys) . ')\b/',
                array($this, 'generateChildDefCallback'),
                $content_model
            );
            //$def->content_model = str_replace(
            //    $this->keys, $this->values, $content_model);
        }
        $def->child = $this->getChildDef($def, $module);
    }