code: function validateUniqueRoot(newRole, respond) {
    // jshint validthis: true
    var self = this;
  
    // attempt to create a root user
    if (auth.roles.isRoot(newRole)) {
      // check for an existing root user
      self.constructor.getRoot(function (err, rootUser) {
        if (err) {
          throw err;
        }
  
        if (rootUser) {
          // found a root user, if it is me everything is ok
          return respond(self.id === rootUser.id);
        }
  
        // there is no root user yet (should only be true once)
        respond(true);
  
      });
    } else {
      // any other role can be set
      respond(true);
    }
  }
docs: Check the uniqueness of the root role (the maximum role)
  @api private
  @param {String} newRole - The role name to save
  @param {Function} respond - The callback function
  @returns {*}