One of more elements required

Hi,

I can't seem to find a solution for the following problem. I hope you
can help me out. Here's a stripped down version of what I've got:

<xsd:complexType name="rootType">
   <xsd:sequence>
      <xsd:element name="platform" type="platformType" minOccurs="0"
maxOccurs="unbounded"/>
      <xsd:element name="device" type="deviceType" minOccurs="0"
maxOccurs="unbounded"/>
   </xsd:sequence>
</xsd:complexType>


Basically, as you can see, the parent element can have zero or more
platform elements AND zero or more device elements.

I'd like to add an additional requirement. The parent element should at
least have one of the two child (platform OR device) elements. How can
this be expressed in XML Schema? Can it be done at all?

Using 'choice' doesn't seem like an option to me since that would only
allow me to include one of the two and not both.

Thank in advance,

Age Bosma