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
  ex a st PSym(p,a) = b
proof
  PSym(p,PSym(p,b)) = b by Th29;
  hence thesis;
end;
