I named it `acts_as_tree_order`, because I thought prefixing it with the gem name would be better than naming it with something like `tree_order` - it would essentially say "this is the ordering that acts_as_tree uses (by default)".

You're right about it being in the public API the way it's written right now. I don't see an easy way to make it "acts_as_tree-private", so I guess I'll just rename it to `default_tree_order` and write a test.