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 _twM16( nA, nB, nC, nD )
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     Define a region of the screen you may be writing to. When the
     mouse cursor is in this region it will be hidden. The coordinates
     refer to the virtual screen.

     To end this function, you must call _twM1() to display the cursor.

       Arguments:

     nA - 'N' Left X coordinate of the region.

     nB - 'N' Upper Y coordinate of the region.

     nC - 'N' Right X coordinate of the region.

     nD - 'N' Lower Y coordinate of the region.

       Return:

     Nothing.

       Example:

     Prepare screen region defined by rows and columns ( 3, 3, 10, 10 )
     for updating with:

     _twM16( 3 * 8, 3 * 8, 10 * 8, 10 * 8 )