Some time ago I wrote a post with a little bit of historical background and an outline of their formal construction: http://advogato.org/person/fxn/diary/475.ht

Some time ago I wrote a post with a little bit of historical background and an outline of their formal construction: http://advogato.org/person/fxn/diary/475.html http://advogato.org/person/fxn/diary/475.html.