code: private synchronized int getStartingMessageId(final Follower follower) {
          if (this.isFollowerActive(follower)) {
              return this.runningFollowerStartMarks.getInt(follower);
          } else if (this.isFollowerTerminated(follower)) {
              return this.terminatedFollowerRanges.get(follower)[0];
          } else {
              throw new IllegalStateException("Follower never before seen.");
          }
      }
docs: For a given follower, return the starting mark.
  
  @param follower
  Tailer in question.
  @return Starting message ID, if after {@link #followerStarted(Follower)}.
  Will throw an exception otherwise.