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 Th2:
  a,b // c,d implies c,d // a,b
proof
  assume
A1: a,b // c,d;
  c,d // c,d by Th1;
  hence thesis by A1,Def1;
end;
