Can this schema be written in XML Schema?

Hi,

I have a simple schema that I wish to use for XML documents.
I've been reading up on XML Schema, trying to find a way of
expressing my schema using it, but so far have failed.  I 
suspect it is currently not possible but thought I would ask 
on this list.  Is it possible, and if not, is XML Schema likely 
to support it at some stage?

Let me explain my schema.  It has a root element called "root".
Every element has precisely one attribute called "is", which 
must take one of three values: "atom", "hash" or "list".  The 
root element must have is="hash", but all other elements can be 
any of the three, the default being "atom".  An atom element
must only have simple content.  A hash element must have element
content only, moreover it must not have more than one child element
of the same name.  A list element must have element content only,
this being elements all called "item" (ie it will have 0 or more
elements, all called "item").

Here is a simple example of a valid xml document:

<?xml version="1.0" encoding="ISO-8859-1"?>
<root is="hash">
  <name>Laura Jones</name>
  <address is="hash">
    <street>123 Trendy Avenue</street>
    <suburb>Prestigious Lakes</suburb>
    <postcode>4321</postcode>
  </address>
  <pets is="list">
    <item>Ginger</item>
    <item>Polly</item>
    <item is="hash">
      <name>Rover</name>
      <breed>Bulldog</breed>
    </item>
  </pets>
  <matrix is="list">
    <item is="list">
      <item>1</item><item>0</item><item>1</item>
    </item>
    <item is="list">
      <item>0</item><item>1</item><item>0</item>
    </item>
    <item is="list">
      <item>1</item><item>0</item><item>1</item>
    </item>
  </matrix>
</root>

What I'd like to do, is to define my schema as a "base schema"
which doesn't say anything about the specifics of a particular
structure, but only enforces the simple rules mentioned above.
The idea would then be to take this base schema and add to
it specific rules (for example about "name", "address", "pets"
and "matrix") to form more tied-down schemas.

Nothing I've read about XML-Schema seems to be appropriate for 
what I'm trying to achieve.  Any help would be much appreciated!

Mark Phillips.