+ EUID (EXCLUSIVE MESSAGE ID'S)
+ -----------------------------
+
+ This is where the groupware magic happens. Any message in any room may have
+a field called the Exclusive message ID, or EUID. We keep an index in the
+table CDB_EUIDINDEX which knows the message number of any item that has an
+EUID. This allows us to do two things:
+
+ 1. If a subsequent message arrives with the same EUID, it automatically
+*deletes* the existing one, because the new one is considered a replacement
+for the existing one.
+ 2. If we know the EUID of the item we're looking for, we can fetch it by EUID
+and get the most up-to-date version, even if it's been updated several times.
+
+ This functionality is made more useful by server-side hooks. For example,
+when we save a vCard to an address book room, or an iCalendar item to a
+calendar room, our server modules detect this condition, and automatically set
+the EUID of the message to the UUID of the vCard or iCalendar item. Therefore
+when you save an updated version of an address book entry or a calendar item,
+the old one is automatically deleted.