Explain model

As I mentioned at the call last week, the SRU document needs an "Explain Model" in the model section. I had drafted one, and thought I had put it in the last draft but somehow it didn't get in and I've lost it. So I've re-drafted it. It needs more work so all of you, feel free to add to it. However, the only purpose I see for it is to provide context for the numerous places within the document where Explain is mentioned. So something like the following should be fairly close to what we need, I think:

Every SRU server provides an Explain file, which contains information about the server’s capabilities. An SRU client may retrieve this file which can then be used to self-configure and provide an appopriate interface to the user.

The Explain file provides such details as query types supported, CQL context sets - and for each context set indexes supported; diagnostic sets, record schemas, sorting capabilities, specification of defaults (need more here)……. . The explain file also includes sample queries, conditions of use (e.g. mandatory display of copyright, syndication rights) (need more here).

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