Conditional Indexes
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     The DBFCDX driver can create indexes with a built-in FOR clause.  These
     are conditional indexes in which the condition can be any expression,
     including a user-defined function.  As the database is updated, only
     records that match the index condition are added to the index, and
     records that satisfied the condition before, but don't any longer, are
     automatically removed.

     Expanded control over conditional indexing is supported with the revised
     INDEX and REINDEX command options as in the new DBFNTX driver.