You must provide a key definition XAB (XABKEY) for each key in an
    indexed file in order to define the key's characteristics. Before
    you create an indexed file, you must establish the contents of
    the XABKEY fields for the primary key and for each alternate key.

    When you invoke an Open or Display service for an existing
    indexed file, you can use XABKEYs if you want to provide your
    program with one or more of the key definitions specified when
    the file was created. Alternatively, the summary XAB provides
    the number of keys, the number of allocated areas, and the prolog
    version assigned to the file.

    The following table lists the symbolic offset, the size in bytes,
    FDL equivalent, and a brief description of each field.

    Offset        Size    FDL Equivalent    Description

    XAB$B_BLN     1       None              Block length
    XAB$B_COD     1       None              Type code
    XAB$L_COLNAM  4       None              Collating sequence name
    XAB$L_COLSIZ  4       None              Collating sequence table
                                            size
    XAB$L_COLTBL  4       COLLATING_        Collating sequence table
                          SEQUENCE          address
    XAB$B_DAN     1       KEY DATA_AREA     Data bucket area number
    XAB$B_DBS     1       None              Data bucket size
    XAB$W_DFL     2       KEY DATA_FILL     Data bucket fill size
    XAB$B_DTP     1       KEY TYPE          Data type of the key
    XAB$L_DVB     4       None              First data bucket virtual
                                            block number
    XAB$B_FLG     1       KEY               Key options flag
    XAB$B_IAN     1       KEY INDEX_AREA    Index bucket area number
    XAB$B_IBS     1       None              Index bucket size
    XAB$W_IFL     2       KEY INDEX_FILL    Index bucket file size
    XAB$L_KNM     4       KEY NAME          Key name buffer address
    XAB$B_LAN     1       KEY LEVEL1_       Lowest level of index
                          INDEX_AREA        area number
    XAB$B_LVL     1       None              Level of root bucket
    XAB$W_MRL     2       None              Minimum record length
    XAB$B_NSG     1       None              Number of key segments
    XAB$B_NUL     1       KEY NULL_VALUE    Null key value
    XAB$L_NXT     4       None              Next XAB address
    XAB$W_POSn    2       KEY POSITION and  Key position, XAB$W_POS0
                          SEGn_POSITION     to XAB$W_POS7
    XAB$B_PROLOG  1       KEY PROLOG        Prolog level
    XAB$B_REF     1       KEY n             Key of reference
    XAB$L_RVB     4       None              Root bucket virtual block
                                            number
    XAB$B_SIZn    1       KEY LENGTH and    Key size XAB$B_SIZ0 to
                          SEGn_LENGTH       XAB$B_SIZ7
    XAB$B_TKS     1       None              Total key field size