Wed Feb 10 09:30:03 CET 2010
Terms do not know of environments
Trying to express the problem of merging environments as a monad lead
me to think that mabye this needs a different terminology.
Terms are self-contained tree structures, and the environment is only
there to track sharing/memoization, i.e. to identify unique nodes.
Let's just call it "Nodes" then.
Next: refactor the code accordingly: the Nodes collection is
parameterized by Term, and only needs some abstract operations on it.