theorem Th67:
  omega in U & a in U & b in U implies b-Veblen a = U-Veblen.b.a
  proof assume
A1: omega in U & a in U & b in U;
    set W = Tarski-Class(b\/a\/omega);
    omega in W & a in W & b in W by Th57,Th66;
    hence b-Veblen a = U-Veblen.b.a by A1,Th64;
  end;
