acl : origin missing from vocab

I noticed that the acl : origin predicate is starting to gain usage :

http://www.w3.org/ns/auth/acl#origin

Yet it doesnt seem to be in the vocab.  Also there seems to be two ways to
actually specify it see :

https://github.com/solid/node-solid-server/blob/master/test/integration/acl-tls-test.js#L282

Should we add it to the vocab, and maybe take a stab at defining it?