how to define a recursive type depth?

Hi.

In an element "interpret" I want to allow normal text and the "b" (bold) "u" (underline) and the "br" tag.

Example:
<interpret>
this is a normal text.
this is a <b>bold</b> text <br/>
this is a <b><u>bold + underline</u></b> text
</interpret>

I have defined the following schema:

....
<xs:complexType name="interpretationsTyp" mixed="true">
 <xs:choice>
  <xs:element name="b" type="interpretationsTyp" minOccurs="0" maxOccurs="unbounded"/>
  <xs:element name="u" type="interpretationsTyp" minOccurs="0" maxOccurs="unbounded"/>
  <xs:element name="br" minOccurs="0" maxOccurs="unbounded">
	<xs:complexType/>
  </xs:element>
 </xs:choice>
</xs:complexType>
....

How can I define the cursion depth? Because the following shouldn't be allowed (which is now available):
<b><u><b><u>text</u></b></u></b>

Thanks
Markus
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