merge-with plus initval

;   Copyright (c) Rich Hickey. All rights reserved.
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;   Eclipse Public License 1.0 (http://opensource.org/licenses/eclipse-1.0.php)
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(defn merge-with*
  "Like merge-with, but handles initial seed value a la reduce.
   Unfortunately not signature compatible with current merge-with."
  ([f maps]
     (when (some identity maps)
       (let [merge-entry (fn [m e]
                           (let [k (key e) v (val e)]
                             (if (contains? m k)
                               (assoc m k (f (get m k) v))
                               (assoc m k v))))
             merge2 (fn [m1 m2]
                      (reduce merge-entry (or m1 {}) (seq m2)))]
         (reduce merge2 maps))))
  ([f initval maps]
     (when (some identity maps)
       (let [merge-entry (fn [m e]
                           (let [k (key e) v (val e)]
                             (if (contains? m k)
                               (assoc m k (f (get m k) v))
                               (assoc m k (f initval v)))))
             merge2 (fn [m1 m2]
                      (reduce merge-entry (or m1 {}) (seq m2)))]
         (reduce merge2 {} (seq maps))))))