David Whitehead (added note; cosmetics) on 16 July 2001@06:54:26. David Whitehead (modified and augmented note; augmented keywords) on 14 April 2004@08:42:25. David Whitehead (another keyword) on 7 October 2005@06:02:58. David Whitehead (tweaked note) on 8 January 2012@09:19:28.