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 Th29:
  PSym(p,PSym(p,a)) = a
proof
  Mid a,p,PSym(p,a) by Def4;
  then Mid PSym(p,a),p,a by Th16;
  hence thesis by Def4;
end;
