code: String eventSubject(long sessionId) {
      if (prefix == null) {
        return String.format("event-%d", sessionId);
      } else {
        return String.format("%s-event-%d", prefix, sessionId);
      }
    }
docs: Returns the event subject for the given session.
  
  @param sessionId the session for which to return the event subject
  @return the event subject for the given session