If the given message priority, PRIOR, and the current message
filtering level (set by MSG_IFSET or MSG_IFGET) indicate that
the message should be delivered, the message text is expanded
and output to the user. If not, the message is discarded.
In either case, any existing message tokens are then annulled.
If the expanded message exceeds the maximum allowed size, it will be
terminated by an ellipsis (i.e. ``...'') but no error results.
If STATUS is not set to SAI__OK on entry, no action
is taken except to annul existing message tokens.
If an output error occurs, an error is reported and STATUS returned
set to MSG__OPTER.