I first went with a custom iterator approach (yielding parent, index, element) to insert at the right position, but since it iterated forward and we had to insert at position or position + 1 it was causing infinite loops.

Instead, I used a recursive approach, and at each layer I iterate on elements backwards to be able to insert in the being-iterated elements. The other option was to iterate on a copy of the list of elements, but this would have been less efficient. reversed is possible here because Element implements both __len__ and __getitem__.

Looks awesome otherwise :)

OK LGTM! Squashing incoming 🙂