Embedding XML with xlink

[PH <p.p.h.vanhagen@siep.shell.com>]

Hello,

I have designed an XML schema that has complexTypes defined for elements
that can be instantiated at multiple levels.
For example:

<xsd:complexType name="A">
</xsd:complexType>

<xsd:complexType name="B">
...
</xsd:complexType>

<xsd:element name="a" type="A"/>

<xsd:element name="b" type="B">
   <xsd:complexType>
     <xsd:sequence>
       <xsd:element name="aa" type="A"/>
       . . .
     </xsd:sequence>
   </xsd:complexType>
</xsd:element>

In the XML instance document I would have the following instantiation:

<a> . . . </a>
<b> <aa> . . . </aa> </b>

Now, I would like to refer back or embed the previous instantiation of
node "a" in node "b". How would I best accomplish that using "xlink"? I
have tried using xlink attributes using xpointer(range-inside(//a)) with
show="embed" but xerces has trouble validating that using the schema I
have.
How can I modify the schema to correctly validate the xlink attributes?
How is this commonly achieved?

TIA

Paul