documentation is missing

I known generally what "hash consing" is
(and that's why I think I might want to use this package)
but I don't see what is the (implied) contract for this implementation:
It looks like I need to write some instances
(to make my type "internable") but what properties should they have,
and what properties of the functions defined in the package
do I get in return?

http://thread.gmane.org/gmane.comp.lang.haskell.cafe/105169/focus=105192