Matrix trace using map-indexed from clatrix library.

gistfile1.clj

(require '[clatrix.core :as cl])
(import '[clatrix.core Matrix])

(defn matrix-trace
  [^Matrix m]
  (->> m
       (cl/map-indexed (fn [i j v] (if (= i j) v 0)))
       (mapv #(reduce + %))
       (reduce +)))