Functionally growing a tree (2): insertion points and zippers

I just uploaded a library that builds on zippers but shifts emphasis from nodes to insertion-points (interstitial positions before, between and after nodes).
It eases growing trees from an empty root.

  ; show-ip represents the currently edited structure with * denoting the insertion-point
  (defn show-ip [ip] (-> ip (insert-left '*) first z/root))
  (def e (-> [] z/vector-zip (insertion-point :append)))
  (-> e show-ip) ; [*]
  (-> e (insert-left 1) show-ip) ; [1 *]
  (-> e (insert-left 1) (insert-right 2) show-ip) ; [1 * 2]
  (-> e (insert-left 1) (insert-right 2) left show-ip) ; [* 1 2]
  (-> e (insert-left [1 2]) show-ip) ; [[1 2] *]
  (-> e (insert-left [1 2]) left show-ip) ; [* [1 2]]
  (-> e (insert-left [1 2]) left right show-ip) ; [[1 2] *]
  (-> e (insert-left [1 2]) left next show-ip) ; [[* 1 2]]
  (-> e (insert-left [1 2]) left next next show-ip) ; [[1 * 2]]
  (-> e (insert-left [1 2]) left next next next show-ip) ; [[1 2 *]]
  (-> e (insert-left [1 2]) left next next next next show-ip) ; [[1 2] *]
  (-> e (insert-left [1 2]) left next next next next prev show-ip) ; [[1 2 *]]