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 Th5:
  a,b // c,d & a,b // c,d9 implies d = d9
proof
  assume a,b // c,d & a,b // c,d9;
  then c,d // a,b & c,d9 // a,b by Th3;
  then c,d // c,d9 by Def1;
  hence thesis by Th4;
end;
