The reference catalog is the new impl. of the Reference Engine. It has
the following key differences.


o References are implmented as "smart pointers" able to do policy during
  relevant operations. 

o References are implmented as real Zope Objects and should be easier to
  migrate from site to site 

o References are keyed on real UUIDs unique in both time and space. 

o Reference can carry arbitrary metadata which can be used to qualify
  the relationships and to impact policy. 

Rock.
