code: protected void prune (long now, List<Entry> history)
      {
          int prunepos = 0;
          for (int ll = history.size(); prunepos < ll; prunepos++) {
              Entry entry = history.get(prunepos);
              if (now - entry.message.timestamp < HISTORY_EXPIRATION) {
                  break; // stop when we get to the first valid message
              }
          }
          history.subList(0, prunepos).clear();
      }
docs: Prunes all messages from this history which are expired.