reserve x,y for set;
reserve X for non empty set;
reserve a,b,c,d for Element of X;
reserve S for OAffinSpace;
reserve a,b,c,d,p,q,r,x,y,z,t,u,w for Element of S;

theorem Th22:
  x,y '||' z,t implies 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;
