XSD problem - variable content in foreign namespace

Hello,

I stumbled upon an XSD modelling problem, which is probably insolvable, but perhaps I overlook something and you might tell me.

The larger topic may be summarized "Modelling elements in foreign namespaces".  Here, "foreign namespace" means a namespace different from the namespace of the root element. The root element namespace I call the local namespace. Accordingly, I speak of local and foreign XSD.

Now the *problem*: How to define multiple content models for an element name in a foreign namespace?

In other words: how to write the XSD so that <x:foo> elements (x denoting a foreign namespace) can have different content dependent on where it appears. In the following example, <x:foo> has different content dependent on if it's a child of <b> or <c>:

How to write XSD reflecting this?

In general, I see two approaches how to model elements in a foreign namespace:

(1) Element reference - e.g. <xs:element ref="x:foo"/>

(2) Imported type - e.g. <xs:element name="foo" type="x:footype"/>

Limitation of (1): the same reference QName means the same thing everywhere, so the document cannot contain <x:foo> elements with different content models, dependent on location.

Limitation of (2): as the type of a complex element describes its *children*, the namespace appears in the *children* of the element with the foreign type, not the element itself. So in the example, I would have to use a type from namespace x in order to describe <x:foo>'s parent - <b> and <c>. This is possible but twisted, as it is more natural to have the type definitions for <b> and <c> in the local XSD. Even worse, it gets really cumbersome when the element in the foreign namespace has siblings in the local namespace, as is the case in the example - note the <y> sibling of <x:foo>. So one has to make a wild construction - writing into the foreign XSD types describing local elements <b> and <c>, which will have to reference local element declarations or element types (example: reference <y>), in order to achieve the mixing of namespaces. Mental ping pong. 

So would you agree that supporting variable content models for foreign element QNames is tricky and not straightforward at all - or do I overlook a simpler possibility?

Thank you for ideas or comments!

Hans-Jürgen

PS: Here come the XSDs which I wrote to model the document:

a.xsd:

x.xsd: