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 Th16:
  Mid a,b,c implies Mid c,b,a
proof
  assume Mid a,b,c;
  then a,b // b,c;
  then b,a // c,b by Th7;
  then c,b // b,a by Th3;
  hence thesis;
end;
