Description

   After you have passed either the file names or the records to
   SORT, call the SOR$SORT_MERGE routine to sort the records. For
   file interface on input, the input files are opened and the
   records are released to the sort. For the record interface on
   input, the record must have already been released (by calls
   to SOR$RELEASE_REC). For file interface on output, the output
   records are reformatted and directed to the output file. For the
   record interface on output, SOR$RETURN_REC must be called to get
   the sorted records.

   Some of the return values are used with different severities
   depending on whether SORT/MERGE can recover. Thus, if you want to
   check for a specific status, you should use LIB$MATCH_COND.