<recordSchema> in the request is a simple string.
It can contain either the short name of a record schema (as mapped in
Explain) or the full identifier.
We could change the semantics to:
It can contain one or more short names or identifiers. If more than one
is supplied and the first is a 'wrapper' schema, then the second and
subsequent should be returned within that wrapper. If the first is not
a know wrapper schema then the server should treat it as a choice of any
of the schemas. [currently an extension]
<recordSchema>mets dc rec</recordSchema>
If the server doesn't support this, it will return a diagnostic saying:
I don't support record schema "mets dc rec".
If the server does support it, but doesn't support one of the individual
schemas, it could return: I don't support record schema "rec"
If it's a choice and the server doesn't support any of the schemas, it
can return multiple diagnostics, one for each.