@CheckResult @NonNull public static Completable updateEmail(@NonNull FirebaseUser user,
      @NonNull String email) {
    return Completable.create(new UserUpdateEmailOnSubscribe(user, email));
  }
@ CheckResult @ NonNull public static Completable updateEmail ( @ NonNull FirebaseUser user , @ NonNull String email ) { return Completable . create ( new UserUpdateEmailOnSubscribe ( user , email ) ) ; }
@see FirebaseUser#updateEmail(String)