@unpublished{Cullingford-etal81,
  author	= {Cullingford, R. E. and Krueger, M. W. and Selfridge, M. and Bienkowsky, M. A.},
  month		= dec,
  year		= {1981},
  title		= {Automated explanations as a component of a computer-aided design system},
  note		= {To appear in \em IEEE Transactions on Systems, Man and Cybernetics.}
}