@incollection{Mitkov90-tutor,
  author	= {Mitkov, R.},
  editor	= {Grabowski, J.},
  year		= {1990},
  title		= {A tutoring System which Explains in Natural Language},
  booktitle	= {BIT-89},
  publisher	= {Akademie Verlag},
  address	= {Berlin},
}