What are clusters?
==================
* a set of connected documents 
* held together by a spanning tree
* clusters have a minimum size (absolute? relative? relative to what?)
* clusters do not break when removing a single bridge (or maybe
  a small amount of bridges)
* ???

What are bridges?
=================
* bridges connect 2 clusters
* of any type except before, sameDate, after, equivalent

What are Hubs?
==============
* Hubs have a lot of outbound associations of directed type
  (except temporal associations - in most cases cites will occure)

What are Authorities?
=====================
* Authorities have a lot of inbound associations of directed type
  (except temporal associations - in most cases cites will occure)

