I was looking for a generic implementation of trees and found Hernan Morales's Trees. From the comment, the package is a "port of Bruce Samuelson's TreeLW implementation. An article on the Tree class hierarchy appeared in the October 1993 issue of The Smalltalk Report."

As it turns out, a copy of that issue of The Smalltalk Report is found on esug.org (in PDF format) and there are many other issues there.