Call SOR$DTYPE to define a key data type not normally supported
    by SORT/MERGE.

    If your SORT/MERGE application needs to compare dates (for
    example) that are stored in text form and that is the only key
    in the records, then use the user_compare argument to SOR$BEGIN_
    SORT or SOR$BEGIN_MERGE. However, if the records contain several
    keys besides the dates in text form, it may be easier to call
    SOR$DTYPE to allocate a key data type code that can then be used
    in the the key_buffer argument to SOR$BEGIN_SORT or SOR$BEGIN_
    MERGE.

    If your SORT/MERGE application has a string key that should be
    collated by a collating sequence defined by the NCS utility, the
    NCS$GET_CS routine can be used to fetch the collating sequence
    definition, and SOR$DTYPE can be called to allocate a key data
    type code for the collating sequence. This key data type code
    can then be used to describe keys that should be compared by this
    collating sequence.