Trees as Linked Lists in Common Lisp

“If you are starting to learn Common Lisp, and have already read the chapter about “Cons” from an introductory book (also called cons cells), then you could try the following exercise. How to represent a tree using nothing but conses; and abstract that implementation with functions to create and traverse the tree.

PLEASE NOTE, that if you need to represent trees in a production program you shouldn’t use lists as described here unless you have a good reason. This is only an exercise in understanding how cons cells work…”

http://gajon.org/trees-linked-lists-common-lisp/