Flattening a tree

Does anyone know how it would be possible to transform a (sub)tree into a
flat list of some of its elements (those considered not "inline"), mixing
cdata and subelements, for example how to transform :

<document>
	<para>
		content-1
		<List>
			<Item>content-2</Item>
			<Item>content-3</Item>
		</list>
		content-4
		</graphic>
		content-5
	</para>
</document>

into :

<document>
	<para>content-1</para>
	<Item>content-2</Item>
	<Item>content-3</Item>
	<para>content-4</para>
	</graphic>
	<para>content-5</para>
</document>

"content-n" being arbitrary contents mixing cdata, entities and inline
elements.

Thanks

Pierre-Yves


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