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@hrefna so, it sounds like you think we should have used multi-typing for actors, or maybe a single Actor root class.

But we didn't. We went with duck typing instead, which seems to work fine. Either of those other ideas would have worked, but this works, too.

If you want to know if you can send an activity to an object's inbox, you check to see if it has an `inbox`. If you want to read its outbox, you check that it has an `outbox`.