reserve AFV for WeakAffVect;
reserve a,a9,b,b9,c,d,p,p9,q,q9,r,r9 for Element of AFV;
reserve AFV for WeakAffSegm;
reserve a,b,b9,b99,c,d,p,p9 for Element of AFV;

theorem Th4:
  a,b // c,d implies b,a // c,d
proof
  assume a,b // c,d;
  then c,d // a,b by Th2;
  then c,d // b,a by Th3;
  hence thesis by Th2;
end;
