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 Th1:
  a,b // a,b
proof
  ex d st a,b // b,d by Def1;
  hence thesis by Def1;
end;
