Clojure's system for creating an ad hoc hierarchy of keywords is familiar to most people who have spent a bit of time with the language. For example, most demos and presentations of the language include examples such as

(derive ::child ::parent)

and they go on to show how this can be used for multi-method dispatch.

In all of the slides and presentations that I've seen, they use the global hierarchy. But it is possible to keyword relationships in custom hierarchies. Some questions, therefore:

• Are there any guidelines on when this is useful or necessary?
• Are there any functions for manipulating hierarchies?

Merging is particularly useful, so I do this:

(defn merge-h [& hierarchies] 
   (apply merge-with (cons #(merge-with clojure.set/union %1 %2) hierarchies))

But I was wondering if such functions already exist somewhere.