XML Schema for unordered elements

Hi,

I would like to know whether it's possible to define an XML Schema for the 
following XML document instance:

<root>
 <a/><b/><c/>
 <k/><k/><k/>
</root>

with the following constraints:
1) <a/>, <b/>, <c/> and <k/> can appear in any order
2) <a/>, <b/> and <c/> can appear at most once under <root>
3) <k/> must appear at least once.

So the following, as well as the example above is also valid:

<root>
  <a/><k/><b/><k/><k/><c/>
</root>

Many thanks for your help in advance
Michael.


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