twMClickShow( nButton, nTop, nLeft, nBottom, nRight )
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     If nButton is pressed in the region defined, display the region in
     the Clipper enhanced colour until the button is released.

       Arguments:

     nButton - 'N'  Mouse button to check.

     nTop,nLeft,nBottom,nRight - 'N' Coordinates of the region to check.

       Return:

     TRUE is there was  a  click in the region.

       Example:

     // Check QUIT
     IF twMClickShow( 1, twRow( 1 ), twCol( 3 ), twRow( 1 ), twCol( 7 ))
         KEYBOARD CHR( K_ESC )
     ENDIF