code: private void addToken(final MutationToken token) {
          if (token != null) {
              ListIterator<MutationToken> tokenIterator = tokens.listIterator();
              while (tokenIterator.hasNext()) {
                  MutationToken t = tokenIterator.next();
                  if (t.vbucketID() == token.vbucketID() && t.bucket().equals(token.bucket())) {
                      if (token.sequenceNumber() > t.sequenceNumber()) {
                          tokenIterator.set(token);
                      }
                      return;
                  }
              }
  
              tokens.add(token);
          }
      }
docs: Helper method to check the incoming token and store it if needed.
  
  Note that the token is only stored if it doesn't exist for the given vbucket already or the given sequence
  number is higher than the one stored.
  
  @param token the token to check and maybe store.