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 Th6:
  for a,b holds a,a // b,b
proof
  let a,b;
  consider p such that
A1: a,a // b,p by Def1;
  b,p // a,a by A1,Th3;
  hence thesis by A1,Def1;
end;
