The task of the FLUSH layer is mainly to respond to a FLUSH event sent from above by coordinating the FLUSH protocol among all group members. This involves broadcasting a FLUSH message to all group members and collecting the responses, possibly resending the FLUSH to a smaller group if some member(s) crashed before receiving all responses. In handling the FLUSH protocol each FLUSH layer needs to return unstable messages. To determine unstable messages, it needs the STABLE layer somewhere underneath it. In case this layer is missing, the set of unstable messages returned will be empty.