Mutable fields are really hard to use correctly in any language, and that's why Clojure encourages the use of immutable data. Thus I find it really odd that the

Mutable fields are really hard to use correctly in any language, and that's why Clojure encourages the use of immutable data. Thus I find it really odd that the article builds a mutable binary tree. The immutable version would have been equally simple. And now you can't put that tree into any of the Clojure's reference types (ref, agent, atom).