[user@ flat_tree]$ vim flat_tree/__init__.py then, down at the bottom ...
#flat_tree = flat_tree_mix_indices_oo flat_tree = flat_tree_append_indices So the test isn't actually using mix_indices. It's instead using append_indices, the older form that is supposed to be more stable. I must have broken it, then resonated discouragement with inhibition etc and failed to continue, had amnesia, etc etc.
