So tries let us associate sequences with values. What data structure lets us 
associate trees with values?

I ask because I am interested in replacing hash consing of expressions with a 
purely functional equivalent so I need a way to map expressions onto 
expressions.

The only idea I've come up with so far is to fold over the tree to make a 
sequence and use that as the key into a trie but it seems a bit naff...

-- 
Dr Jon D Harrop, Flying Frog Consultancy Ltd.
http://www.ffconsultancy.com/products/?e