[nicoburns]

> If you "just" rely on them not to do that, then your internals will effectively be frozen forever.

Or they will be broken when you change them and they upgrade. The JavaScript ecosystem uses this convention and generally if a field is prefixed by an underscore and/or documented as being non-public then you can expect to break in future versions (and this happens frequently in practice).

Not necessarily saying that's better, but it is another choice that's available.