One way is not to use a tree structure but to aggregate by "composed" keys,
starting with [:attr1] then [:attr1 :attr2] ...
(defn sum-by [data attrs]
(let [aggregated (group-by (apply juxt attrs) data)]
(zipmap (keys aggregated) (map #(reduce + (map :mv %)) (vals
aggregated)))))
(println (sum-by data [:attr1 :attr2]))
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