@incollection{Plum87,
  author	= {Plum, Guenter A. and Cowling, Anne},
  editor	= {Steele, Ross and Threadgold, Terry},
  year		= {1987},
  title		= {Social constraints on grammatical variables: tense choice in English},
  booktitle	= {Language topics. Essays in honour of Michael Halliday},
  publisher	= {Benjamins},
  address	= {Amsterdam}
}