(insert-child loc item)
Inserts the item as the leftmost child of the node at this loc, without moving
(use 'clojure.zip) ;;warnings OK
(def zp (zipper vector? seq (fn [_ c] c) [[1 2 3] [4 [5 6] 7] [8 9]]) )
(root (insert-child (-> zp down right) 42))
=>([1 2 3] (42 4 [5 6] 7) [8 9])
(def zp2 (zipper vector? seq (fn [_ c] c) [74 75])
(root (insert-right (-> zp down right) zp2))
=>([1 2 3] [4 [5 6] 7] [[74 75] nil] [8 9])
(root (insert-left (-> zp down right) zp2))
=>([1 2 3] [[74 75] nil] [4 [5 6] 7] [8 9])