@incollection{Halliday76-tense,
  author	= {Halliday, Michael A. K.},
  editor	= {Kress, Gunther R.},
  year		= {1976},
  title		= {The English verbal group},
  booktitle	= {Halliday: system and function in language},
  publisher	= {Oxford University Press},
  address	= {London}
}