_twAttrChg( expC, [expN] )
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     Change the color attribute in a Clipper SAVESCREEN variable.

       Arguments:

     expC - 'C' SAVESCREEN Variable.

     expN - 'N' New DOS colour attribute.

       Return:

     SAVESCREEN with all attribute bytes changed.

       Example:

     sScr := _twAttrChg( SAVESCREEN( 5, 5, 10, 10 ), 8 )
     Changes the colour of sScr to GREY (ASCII 8)

     NOTE: This function was used to get around a problem with
           TRANSFORM() in Clipper 5.01 which shows up occasionally. I
           didn't feel like finding out when.