Find posts about post

It could be useful if the server could provide information about other posts in the users collection that link to a specific one, e.g. to easily compile information like "5 people you follow liked this". This could be related to [Indicating whether posts have already been responded to](https://github.com/indieweb/microsub/issues/3) or part of [Searching for Content](https://indieweb.org/Microsub-spec#Searching_for_Content)

Start by reading issue #3, “Indicating whether posts have already been responded to,” and the “Searching for Content” section of the Microsub specification. Determine how this request relates to those existing ideas and what server-provided information is required. Done means the specification has a decided, testable description of the behavior.

Written by the indexing model from the issue text.