This comes out of discussions around the semantics of the http://librarysimplified.org/terms/rel/revoke link relation:

• Change the circulation manager to support POST and DELETE on the link relation, per the spec.
• Change the clients to handle the case where a book has no "revoke" link. For books where the client knows how to return the book on its own (books obtained through ACS), the end-user experience will be unchanged. For books where the client doesn't know how to do this (Axis 360 audiobooks), the client needs to give some indication that this book can't be returned manually.
• Once the mobile app changes are deployed, change the circulation manager to omit the link with rel="http://librarysimplified.org/terms/rel/revoke" if it knows it won't be able to actually carry out a return for a book on loan. This means adding a capability to BaseCirculationAPI so that each vendor API can explain whether or not it supports returns.
• Add an appropriate "type" to the revoke link to make it clear that the document returned on success is an OPDS entry. This should also be added to the spec as an expectation.