graph.clj

(defn- merge-edges [over-layer incoming? edges-seq]
  (->> (for [[i edges] (indexed (reverse edges-seq))
             [id edge] edges]
         (let [pos  (merge-position over-layer id)
               head (or (merge-head over-layer id) id)]
           [(if incoming? [pos i], [i pos])
            {head edge} id]))
       (sort-by first #(compare %2 %1))
       (map second)
       (apply merge-with adjoin)))

(defn merge-nodes [over-layer keyseq nodes]
  (if-let [ks (edges-keyseq keyseq)]
    (let [edges (merge-edges over-layer
                             (= :meta (first keyseq))
                             (map #(get-in % ks) nodes))]
      (-> (reduce adjoin nil
                  (map #(dissoc-in* % ks) nodes))
          (given (seq edges)
                 (assoc-in* ks edges))))
    (reduce adjoin nil nodes)))