Two way relationship with immutable data structure.

tow-way.clj

(defprotocol XmlNode
  (get-parent [_])
  (get-children [_]))

(def empty-node
  (reify XmlNode
    (get-parent [_] nil)
    (get-children [_] '())))


(defn- reasign [parent children]
  (map #(reify XmlNode
           (get-parent [_] parent)
           (get-children [this] (reasign this (get-children %))))
       children))
  

(defn add-child [parent child]
  (reify XmlNode
    (get-parent [_] (get-parent parent))
    (get-children [this] (reasign this (concat (get-children parent) [child])))))