+ * @section s4u_mb_matching How are sends and receives matched?
+ *
+ * The matching algorithm is as simple as a first come, first
+ * serve. When a new send arrives, it matches the oldest enqueued
+ * receive. If no receive is currently enqueued, then the incomming
+ * send is enqueued. As you can see, the mailbox cannot contain both
+ * send and receive requests: all enqueued requests must be of the
+ * same sort.
+ *