The SOR$BEGIN_MERGE routine initializes the merge process by
    passing arguments that provide the number of input streams, the
    key specifications, and any merge options.

    You must define the key by passing either the key buffer address
    argument or your own comparison routine address. (You can also
    define the key in a specification file and call the SOR$SPEC_FILE
    routine.)

    The SOR$BEGIN_MERGE routine initializes the merge process in
    the file, record, and mixed interfaces. For record interface
    on input, you must also pass the merge order, the input routine
    address, and the longest record length. For files not on disk,
    you must pass the longest record length.

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