@mike

Which is it? If it's both, how does that work?

Yes, it's both. The backfilling algorithm described in the FEP should be used on a top-level post. When this post has a contextHistory property, it will resolve to a collection of activities. When it has a context property, it will resolve to a collection of posts.

wtf is a "collection of posts"

Alright, I will add a definition to the FEP.

"post" is any kind of attributed object. Usually it's a Note or an Article, but it could be an Event too, or something else entirely. What's important is that "post" is not an activity.

And if "contextHistory" is an equal player with "context", every.single.fediverse.project now has to examine every context they copy to their own objects and activities, just to ensure they get the right one in each case. And if both are present, copy both - after checking that "context" is or isn't the one they really want.

It shouldn't be a problem because with the proposed backfilling algorithm the collection item type is always known beforehand.

If it's a problem, you can use duck typing on collection items. That's what I do when I fetch an object whose class is not known.