code: function isValid(app) {
    if (isValidApp(app, 'base-helpers', ['app', 'views', 'collection'])) {
      debug('initializing <%s>, from <%s>', __filename, module.parent.id);
      return true;
    }
    return false;
  }
docs: Return false if `app` is not a valid instance of `Base`, or
  the `base-helpers` plugin is alread registered.