References to a value of an element in another element?

Greetings,
         I have a simple question, but i am not sure it is possible to
represent  it in XMLSchema, if so any help would be really appreciated.

I have variable-length structure and the length of the 2nd element depends on
the value of the first element. How is it possible to specify this in
XMLSchema?

<xsd:complexType name="Chunk">
    <xsd:sequence>
      <xsd:element name="size"   type="xsd:integer"/>
      <xsd:element name="data"  type="xsd:hexBinary" ?????/>
    </xsd:sequence>
  </xsd:complexType>

The problem is with element "data", how do i specify that its length is the
value "size"???

Thank you very much,
Carl Eric Codère



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