say some random person sees the statement above. they don't know who john or sally are, and they don't know what "knows" means in this context.
well, if we do a little work upfront, they actually *can* know what all of these terms mean, **without ever asking us directly**
we put a resource on example.com for each of these terms, and each resource describes the subject of that identifier -- it is a "resource descriptor".
the resource for knows/1 can define itself explicitly with a schema
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