reserve a,b,c,d for Ordinal;
reserve l for non empty limit_ordinal Ordinal;
reserve u for Element of l;
reserve A for non empty Ordinal;
reserve e for Element of A;
reserve X,Y,x,y,z for set;
reserve n,m for Nat;
reserve f for Ordinal-Sequence;
reserve U,W for Universe;
reserve F,phi for normal Ordinal-Sequence of W;
reserve g for Ordinal-Sequence-valued Sequence;

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;
