Finally got addition of categories to category trees working. Involved transforming the on disk CouchDB data structure, vectors of maps of vectors of maps..., which is the same as the JSON tree returned in the REST API, to a data structure more amenable to manipulation, which is ordered maps of maps of ordered maps of maps... and then back again once the manipulation is complete.
Worked out the remaining tasks to finish up Clojure API and unit testing for taxonomy and categories.