• A diagram, even if not pretty, would be great.

• Where does the local cache live? In an app dir? (I guess so, and wonder whether it should be made clearer)

• Regarding register, since it's meant to be run interactively I wonder whether it should behave like usethis' functions with cli messages.
  e.g. check-mark, registered dataset
  check-mark, registered with Hash Archive
  todo, save the hash and use it in e.g. resolve()

• should query_sources() have some custom printing "An registered content with hash and X sources"?

• should query_sources() have a verbose mode where it says what it's querying, like what we have in dev codemetar? "querying hash archive... querying local cache at ... querying Software Heritage"

• if the sources live in an app dir, it means that if you move your Rproj or somewhere else, you lose the sources, or can the sources be cached in a project directory too?

• " This is because in addition to maintaining a local registry of sources, contentid registers online sources with the Hash Archive, hash-archive.org. " very cool but should a warning be added that it means you shouldn't e.g. use the raw URL to a GitHub private repo because that thing contains a token?

to be continued...