for S being OAffinSpace
for x, y, z, t being Element of S st x,y '||' z,t holds
( x,y '||' t,z & y,x '||' z,t & y,x '||' t,z & z,t '||' x,y & z,t '||' y,x & t,z '||' x,y & t,z '||' y,x ) by Th2;