The SOR$END_SORT routine ends a sort or merge operation, either
    at the end of a successful process or between calls because of an
    error. If an error status is returned, you must call SOR$END_SORT
    to release all allocated resources. In addition, this routine can
    be called at any time to close files and release memory.

    The value of the optional context argument is cleared when the
    SOR$END_SORT routine completes its operation.

    Some of the following condition 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.