Conditionally Validate a dbf index key value as UNIQUE
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 Syntax

            IsKeyValid( <xKey>, <nMode>, <cAlias>, ;
                                                    <bLookup>, <bAssign> ) -> lOk

 Arguments:

                <cKey>       is a valid key value for the current index.

                <nMode>  is a manifest constant to conditionally validate the key

                <cAlias> is the file alias to validate against

                <bLookup> is a code block to display a lookup/picklist

                <bAssign> is a codeblock to assign fields to variables

 Returns:

                A logical value, .T. if the key is conditionally valid

 Description:

                GT_ISKEYVALID() can be used to validate a get against an indexed file,
                                   to ensure a UNIQUE key value.

                to validate a new record as UNIQUE,
                to validate an edit of an existing record as UNIQUE,
                optionally providing a picklist codeblock on failure,
                optionally providing an additional assign codeblock on success.

 Examples:


 #define NEWRECORD  1
 #define DISPEDIT   2
 #define DISPONLY   3
 #define LOOKUP   4

************************
        PROCEDURE test()

           cVatId := "  "
           nVat   := 0.0

           FirstGet( NEWRECORD )
           READ

           SecondGet( LOOKUP )
           READ

        RETURN
*************************
 function FirstGet( nMode )

         @ 08, 20 SAY "Input a New VAT Code   : " GET cVatId    PICTURE "@K!" ;
           VALID ISKEYVALID( cVatId, nMode ) ;
                         when ( nMode == NEWRECORD )

**************************
 function SecondGet( nMode )

         @ 10, 20 SAY "Input current VAT Code : " GET cVatId PICTURE "@K!" ;
                                  VALID ISKEYVALID( cVatId, LOOKUP, "VAT", ;
                                                                        {|| vat( @cVatId, @nVat ) }, ;
                                                                        { || nVat := vat->Vat } )

         @ 11, 20 GET nVat PICTURE "99999.99" when ( .F. )

        return ( NIL )
***************************
    where : {|| vat( @cVatId, @nVat ) }
           is a codeblock calling a picklist which can assign variables
           passed by reference.

    and   : { || nVat := vat->Vat } )
           is a codeblock which will assign ALIAS->FIELD to the
           looked up variable refered to.


 Source: GT_VALID.PRG

 Author:  George Brennan