I'd like to point out here that these two problems -- optimizing delivery by server rather than by actor, and widely propagating public activities -- are related but not identical.
In the development of ActivityPub, we originally had an endpoint called `publicInbox` for the second problem -- propagating public activities. But when we started working on the optimization issue, we defined a `sharedInbox` also. Here's the key point: we actually combined both features in one endpoint.
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