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   For N:1 relations, the referential integrity rules cannot be applied. If
   for any reason, the referential intergity is to be set, an inverse 1:N
   relation must be created between the two files. To do this, it is
   necessary to define an index with the relational field in both files as
   indexkey.

   In case that the Lookup file is a Table file, it is necessary to convert
   this file into a Standard file. This is a necessary step, because Table
   file can only be terminal files in relation branches.

Example:

   A database has 2 files:

     . WBHea (Way-bill header) with fields WBCod, CusCod,
       WBDate, etc, and index on WBCod (WBHea1) and CusCod (WBHea2).

     . Cus (Customers) with fields CusCod, CusAdd, CusZIP, etc,
       and index on CusCod (Cus1).

   Between the WBHea and the Cus file, a Lookup relation has been set using
   the CusCod field as the common field and the Cus1 index. In this case,
   though, the referential integrity is not guaranteed. If this function is
   necessary, both files must have an inverse relation set. Using the second
   index of WBHea, WBHea2, a 1:N relation is to be created between Cus and
   WBHea.

     Parent  Type   Child    Index     Key
     ------------------------------------------------
     WBHea   N:1    Cus      Cus1      WBHea->CusCod
     Cus     1:N    WBHea    WBHea2    Cus->CusCod