@Article{	  dixkum93,
  author	= {J\"{u}rgen Dix and Martin Kummer},
  title		= {{H}erbrand's {T}heorem for {N}onstandard {I}nference
		  {O}perations},
  journal	= {Journal of Logic and Computation},
  year		= {1994},
  volume	= {4},
  number         ={2},
  pages		= {143--156}
}