XABSUM

   The summary XAB (XABSUM) can be associated with a FAB at the time
   a Create, Open, or Display service is invoked. The presence of
   this XAB during these calls allows RMS to return to your program
   the total number of keys and allocation areas defined and the
   version number when the file was created. Note that a XABSUM is
   used only with indexed files.

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

   Offset      Size     Description

   XAB$B_BLN   Byte     Block length
   XAB$B_COD   Byte     Type code
   XAB$B_NOA   Byte     Number of allocation areas defined for the
                        file
   XAB$B_NOK   Byte     Numbers of keys defined for the file
   XAB$L_NXT   Longword Next XAB address
   XAB$W_PVN   Word     Prolog version number