/**
  * Sets the renderer for markdown fields to use
  * @param {string} $renderer Class Name of an implementation of IMarkdownRenderer
  */
 public static function setRenderer($renderer)
 {
     if (ClassInfo::classImplements($renderer, 'IMarkdownRenderer')) {
         self::$renderer = $renderer;
     } else {
         user_error('The renderer ' . $renderer . ' does not implement IMarkdownRenderer', E_USER_ERROR);
     }
 }