Dave Lemke and Nathan Meyers gave me many helpful explanations.

The following people contributed code, bug reports and/or bugfixes:
Huw D. M. Davies, Gordone Deane, Ralf Deininger, Michael Hoegeman,
Pavel Kankovsky, Stephen Montgomery-Smith, Chris Webb, Keith White,
Robert Wilhelm, David Wragg, Joerg Pomnitz, David Dawes, 
Toshiki Atsushiba.
