>># 
>># Copyright 1990, 1991 by the Massachusetts Institute of Technology and
>># UniSoft Group Limited.
>># 
>># Permission to use, copy, modify, distribute, and sell this software and
>># its documentation for any purpose is hereby granted without fee,
>># provided that the above copyright notice appear in all copies and that
>># both that copyright notice and this permission notice appear in
>># supporting documentation, and that the names of MIT and UniSoft not be
>># used in advertising or publicity pertaining to distribution of the
>># software without specific, written prior permission.  MIT and UniSoft
>># make no representations about the suitability of this software for any
>># purpose.  It is provided "as is" without express or implied warranty.
>># 
>># $XConsortium: sbtrctrgn.m,v 1.7 92/06/11 17:42:44 rws Exp $
>># 
>>#
>>TITLE XSubtractRegion CH10
>>ASSERTION A XSubtractRegion-1
A call to XSubtractRegion subtracts the region srb from
region sra and stores the result in region dr_return.
>>STRATEGY
Create a region using XCreateRegion.
Set the region to a rectangle using XUnionRectWithRegion.
Subtract an encompassing region from itself using XSubtractRegion.
Verify that the result is the empty region using XEmptyRegion.
Subtract an enclosed region from and encompassing one using XSubtractRegion.
Verify that the result is not the empty region using XEmptyRegion.
Combine this result with the enclosed region.
Verify that the result is the original region.
