I've never been very clear on how best to do stuff like this.

From my experiments, `private` seems to "cancel out" the effect of the earlier `module_function`, so I either need to add another `module_function` specifier after the `private`, or explicitly mark just this one method as a `module_function`.

I assume you'd like either of those better than the `self.` I've used here.  Do you have a preference?