Updating trees (was Re: (no subject))

[Mark Carroll]

As well as the specific help you've already gotten, I'll throw in a
recommendation for Chris Okasaki's "Purely Functional Data Structures"
book which gives a general course in how to to design and implement
standard data structures in a functional language. He doesn't give all the
answers; instead, he explains how to figure them out yourself, which is
presumably both more painful and more edifying. (-:

-- Mark