reference to element, elementFormDefault unqualified

Hi,

I'm experiencing a behavior with a referenced element that I don't quite
understand. I have a schema for common datatypes that defines an own
targetNamespace. Within, I have a global element declared that actually is used
by a complexType  definition in the same schema, but that can also be
independently used by an importing schema.

Then I have a main schema that imports the common schema and defines two
elements, one of the complex type and one referencing the global element.

To hide namespaces from instance documents, elementFormDefault is set to
unqualified.

common schema:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
targetNamespace="nscommon" xmlns="nscommon" elementFormDefault="unqualified">
    <xs:complexType name="LinkType">
        <xs:sequence>
            <xs:element name="sub1" type="xs:string"/>
            <xs:element ref="sub2"></xs:element>
        </xs:sequence>
    </xs:complexType>
    <xs:element name="sub2" type="xs:string"/>
</xs:schema>

main schema:

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
targetNamespace="nsprocess" xmlns="nsprocess" xmlns:common="nscommon"
xmlns:process="nsprocess" elementFormDefault="unqualified">
    <xs:import namespace="nscommon" schemaLocation="common.xsd"/>

    <xs:element name="root" type="process:rootType"/>

    <xs:complexType name="rootType">
        <xs:sequence>
            <xs:element name="link" type="common:LinkType"/>
            <xs:element ref="common:sub2"/>
        </xs:sequence>
    </xs:complexType>
</xs:schema>

Now the following instance document is invalid, since the processor seems to
expect a namespace declaration (nscommon) for the sub2 elements.

instance document:

<process:root xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:process="nsprocess"
    xsi:schemaLocation="nsprocess process.xsd">
    <link>
        <sub1/>
        <sub2/>
    </link>

    <sub2/>

</process:root>

However, when I set elementFormDefault to qualified in both schemas and qualify
all elements in the instance document with the appropriate namespace, the
document is valid.

Now why is this thing behaving differently, and is there a way of making such a
reference with elementFormDefault unqualified at all?

cheers,
Oli

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