paths.clj

(defn paths
  ([root] (paths [] root))
  ([parent x]
   (if (map? x)
     (mapcat (fn [[k v]] (paths (conj parent k) v)) x)
     [parent])))

(def m
  {:q {:z 1}
   :x {:y {:a 1}
       :b {:b 2}}})

(paths m) ;=> ([:q :z] [:x :y :a] [:x :b :b])

(reduce #(update-in %1 %2 dec) m (paths m))
;=> {:q {:z 0}, :x {:y {:a 0}, :b {:b 1}}}