Some bits of Overton's mode system for Mercury

The beginnings of a re-implementation of Mercury's mode system, as described
in: David Overton. Precise and Expressive Mode Systems for Typed Logic
Programming Languages.  University of Melbourne, Department of Computer
Science and Software Engineering.  Ph.D. thesis. December, 2003.
<http://www.mercury.csse.unimelb.edu.au/information/papers.html#dmo-thesis>