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Apologies as I'm not sure if the problem is with clj-stacktrace or with https://github.com/purcell/ac-nrepl, but it's a bit of a showstopper for using this in the REPL in emacs so thought I'd mention it.
After substituting (use clj-stacktrace.repl) for (use 'clojure.stacktrace) in my .lein/profiles.clj :repl-options :init, ac-nrepl suddenly starts to hang when I nrepl-jack-in on "Caching JVM class names". Only restarting emacs fixes it.
Using lein 2.0.0 with latest nrepl.el and ac-nrepl.