dtd limit depth of elements

Hy,

again another dtd question.

I have a sect element, which I would like to have only 3 levels of depth inside 
itself.
And in this section and the 2 under the first there can be other elements like 
list,para and so on.

So that this xml code should be valid:
<sect>
        <sect>
                <sect>
                        <list>
                                <entry>test<entry>
                        </list>
                </sect>
                <para>bla</para>
        </sect>
        <para>next bla</para>
</sect>

But another sect inside the third has to be invalid!
So, how can i limit the depth of elements inside a dtd?

I had a look at the xml-dev maillist archive but didn't find an example, so I 
hope someone in this list knows it.
Thanks,again :-)

Homer30s


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