twRestShadowArea( tShadow )
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     If a window has a shadow region, this function will eliminate the
     shadow and restore the underlying screen area in correct colours.

       Arguments:

     tShadow - 'N' Shadow Type

       Return:

     Current Window Level

       Example:

     twRestShadowArea( 3 )

     Restores screen overwritten in the shadow attribute in the
     original colours for a 3 shadow type. Necessary coordinates are
     determined from the stack.