Schema attr on instance

Would be nice to have a schema attribute on instance so that you could better control the schema that are deemed to be "applicable" to that instance's data nodes.

<instance schema="#s1 http://example.org/s2.xsd"> ... </instance>

This way, your model could have 20 schemas, but the other 18 would not be applicable.

One would think that, by default, if no schema attribute is given, then only the schema with the same target namespace as the root element should be applied.