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ddPkChk()
Verifies uniqueness of an expression
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Syntax:

     ddPkChk( <nNdxOrde>, <cKeyExpr>,;
                     [<cMessage>] ) --> lSuccess

Parameters:

     <nNdxOrde> Index order on which to verify uniqueness
     <cKeyExpr> Espression to validate
     <cMessage> Optional error message in case of duplicates

Returns:

     <lSuccess> If .T., the expression is unique

Description:

     This function verifies uniqueness of the primary/unique keys, i.e.
     checks if the expression entered is not already stored in the files.
     If the result is True, the expression is not found in the file, thus
     the key not duplicated.

Example:

     IF !Inv->(ddPkChk( 1, tbPkExp( oWin ) ))
        // Loop
     END

See also:

     dfPkNew()