>># 
>># 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: xrrgn.m,v 1.5 92/06/11 17:44:09 rws Exp $
>># 
>>#
>>TITLE XXorRegion CH10
>>ASSERTION A XXorRegion-1
A call to XXorRegion computes the difference between the
union and intersection of the regions sra and srb and stores
the result in dr_return.
>>STRATEGY
Create regions R1 R2 R3 Ru Ri Rx using XCreateRegion.
Set R1 to a polygon using XPolygonRegion.
Set R2 to a polygon which partially intersects R1 using XPolygonRegion.
Set Ru to the union of R1 and R2 using XUnionRegion.
Set Ri to the intersection of R1 and R2 using XIntersectRegion.
Set R3 to the difference between Ru and Ri using XSubtractUnion.
Set Rx to the difference between the union and intersection of R1 and R2 
  using XXorRegion.
Verify that Rx is the same as R3 using XEqualRegion.
