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 twNthOccur( cSource, cTarget, nWhich, lCase )
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     Find the position of the Nth occurrence of a string in a target
     string.

       Arguments:

     cSource - 'C' String to search for.

     cTarget - 'C' String to search.

     nWhich  - 'N' Number of the occurrence to find.

     lCase   - 'L' Logical for case sensitive search. The default is
                   .F.

       Return:

     The position in the string cTarget of the nth
     occurrence of cSource or zero if not found.

       Example:

     twNthOccur( "ab", "abracadabra", 2 ) -> 8