[XML Schema 1.1] Using doc() in xs:assert ... the referenced document needs a schema?

Hi Folks,

Here's a simple schema that uses xs:assert to check that the value of the <country> element matches one listed in an instance document (countries.xml):

---------------------------------------------------------------
<?xml version="1.0"?>
<schema xmlns="http://www.w3.org/2001/XMLSchema"
        elementFormDefault="qualified">

    <element name="Example">
        <complexType>
            <sequence>
                <element name="country">
                    <simpleType>
                        <restriction base="string">
                            <assert test="$value = doc('countries.xml')//country" />
                        </restriction>
                    </simpleType>
                </element>
            </sequence>
        </complexType>
    </element>

</schema>
---------------------------------------------------------------


While running this example, I discovered these requirements:

   - the instance document referenced by xs:assert (countries.xml) 
     must have a schema, and 

   - I have to import that schema. 

Why?

These requirements don't seem reasonable to me. 

I should be able to reference any instance document (under my control or otherwise) and use it as a data source.

These requirements mean:

   - I can't reference instance documents that don't have a schema

   - I can't reference instance documents that uses some other
     schema language, such as DTD or RELAX NG.

   - Everything is tightly coupled: the schema, the referenced 
     instance document and its schema. I can't easily switch to 
     another instance document and use it as a data source.


Would someone please explain the rationale for these requirements?

/Roger