reserve AFV for WeakAffVect;
reserve a,b,c,d,e,f,a9,b9,c9,d9,f9,p,q,r,o,x99 for Element of AFV;

theorem Th3:
  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;
