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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.