DOCBOOK: Problem with simplesect within section

Hi,

taking the simplesect example from TDG and slightly modifying it I get:

> cat test.xml
<?xml version='1.0'?>
<!DOCTYPE section PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
          "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<section><title>Additional Coding</title>

<para>
Support for the additional features requested will be provided.
</para>

<simplesect><title>Estimated Time</title>

<para>
2 to 3 weeks.
</para>

</simplesect>
</section>

Asking nsgml about the correctness of this file, I get:

> nsgml -s xml.dcl test.xml
nsgmls:test.xml:10:11:E: document type does not allow element "simplesect" 
here

The original example from TDG using sect1 instead of section works well.

Is this an error on my side in understanding TDG or is it a problem within 
DocBook?

Greetings,

Norbert Eicker