Conditional Indexes
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     Conditional indexes are produced by using a FOR condition in the index
     creation process.  These indexes are made fully maintainable by storing
     the FOR condition in the index header.  This condition is subsequently
     retrieved and compiled each time the index in opened.  During updates,
     items are added to the index only if they meet the criteria of the
     condition.

     Since older applications do not have the ability to recognize and use
     the condition stored in the header, they must be prevented from opening
     the index since they corrupt the index.  This is accomplished by
     modifying the signature of the index (in the index header) preventing
     pre-CA-Clipper 5.2 applications from being able to open the index.
     CA-Clipper 5.2 applications can detect the flag in the header and will
     automatically use the stored FOR condition correctly.