def [](id)
      return @users[id] if id.start_with? '@'

      res = @users.find { |_, u| u.displayname == id }
      res.last if res.respond_to? :last
    end
def [](id)
      return @users[id] if id.start_with? '@'

      res = @users.find { |_, u| u.displayname == id }
      res.last if res.respond_to? :last
    end
Initializes a new Users instance.
 Gets a user by ID or display name.

 @param id [String] A user's ID or display name.
 @return [User,nil] The User instance for the specified user, or
   `nil` if the user could not be found.