T is worth starting off with a description of the some recent and relevant developments which were made before September 2003. The following few paragraphs summarise and shed light at some of the main principles. These principles consequently describe some methods which are still used in the existing system - systems that surely needed to be extended and their understanding was the most crucial.
Smith's work follows the work of Davies in a more-or-less obvious sense, but it explores a different domain with slightly different aims. Each of these two research efforts will be dealt with in turn.