Change Log

Major update published on 16 June 2003

Global description:

Article has been substantially revised and updated. New Section 4 and various Subsections (2.3 – 2.5, 2.7.5 – 2.7.7, 2.8, 2.10) have been added. The number of references (342) has more than doubled.

Previous version:

http://www.livingreviews.org/lrr-1998-8