code: public long addPropose(MessageID messageID, Address from, long sequenceNumber) {
          MessageInfo messageInfo = sentMessages.get(messageID);
          if (messageInfo != null && messageInfo.addPropose(from, sequenceNumber)) {
              return messageInfo.getAndMarkFinalSent();
          }
          return NOT_READY;
      }
docs: Add a propose from a member in destination set
  @param messageID         the message ID
  @param from              the originator of the propose
  @param sequenceNumber    the proposed sequence number
  @return NOT_READY if the final sequence number is not know, or the final sequence number