CMIS XII: Must-understand signaling in app:accept

Since rudimentary AtomPub clients may be unduly surprised when a sophisticated CMIS AtomPub server summarily rejects a POST request with a hard to understand response. So, it is best to forewarn such clients about the requirements of the server.

To do this, one would need to use a must-understand signaling mechanism in AtomPub, of which there is exactly ONE in standard AtomPub. The app:accept element's content signals to the client what type of content the server is willing to accept. If the request fails because the content does not meet the expectations of the server, a 415 response is generated. This means the rudimentary client can easily understand the sophisticated server. Plus, the sophisticated server now has a means of conveying additional information to the sophisticated client. And, best of all, no permission is needed from anyone at IETF, unlike the registration for "type", although an IANA registration for the parameter would be good to have.