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 _twM8( nC, nD )
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     Set mouse cursor maximum and minimum vertical position. You may
     limit the mouse cursor to a range of screen columns with this
     function.

       Arguments:

     nC - 'N' Minimum screen Y coordinate in virtual screen coordinates.

     nD - 'N' Maximum screen Y coordinate in virtual screen coordinates.

       Return:

     Nothing.

       Example:

     Limit mouse cursor to rows 3 to 10 with:

     _twM8( 3 * 8, 10 * 8 )