@article{urqu:four07,
  author	= {Alasdair Urquhart},
  title		= {Four Variables Suffice},
  journal	= {The Australasian Journal of Logic},
  year		= 2007,
  volume	= 5,
  pages		= {66-73},
  URL		= {http://www.philosophy.unimelb.edu.au/ajl/2007/},
}