msg — A message in a message set
In a MsgSet, each MsgEntry contains at least
one Msg.  A Msg consists of a main message
(MsgMain), 
and optionally one or more submessages (MsgSub) 
or related messages (MsgRel).
Additional information or explanation for the message is contained
in the siblings of Msg within the MsgEntry.
See MsgSet.
Formatted as a displayed block. Sometimes suppressed.
On the whole, the semantics of MsgSet
are not clearly defined.