This example was confusing at first because the element the message
indicated wasn't in the list of possible values was but the possible
values didn't match up either. This ensures the example is consistent
with the logic being presented.
Hydra passes the full revision in to the input, which we pass through.
If we don't get this ,we try to get it from other sources, or default to
master which should have the definition in a close-ish location.
All published docs should have theURL resolve properly, only local
hackers will have the link break.