public List<NotificationType> getNotificationTypes(User user)
 {
     // ...
     // end::UserService[]
     return new List<NotificationType>();
     // tag::UserService[]
 }
 public void unregisterForNotifications(User user, NotificationType type)
 {
     // ...
 }
 public void register(User user, NotificationType type)
 {
     // ...
 }
 public void blockUser(User user)
 {
     // ...
 }
 // ...
 // end::UserRestAPI[]
 public string ToJson(User u) {
     return "";
 }