How to build a tree from scratch if you can't insert on leaf nodes?

[Overdr0ne]

If I try to insert a child on a leaf node, from a list zipper, treepy throws an error: "Called children on a leaf node", e.g.

(let* ((l '())
      (z (treepy-list-zip l))
      (z (treepy-insert-child z 1))
      (z (treepy-insert-child z 2))
      (z (treepy-down z))
      (z (treepy-insert-child z 3))
      )
  z)

Your tests show how to use a zipper created from an existing list. The much more common case would seem to be you start with an empty zipper, then add and remove things as needed. But how can you do that if you can't insert on a leaf node? How does clojure handle that?