<element xmlns="http://relaxng.org/ns/structure/1.0" name="device">

    <choice>

      <group>

	<attribute name="link">

	  <value>ethernet</value>

	</attribute>

	<optional>

	  <element name="ethernet-element">

	    <text/>

	  </element>

	</optional>

      </group>

      <group>

	<attribute name="link">

	  <value>serial</value>

	</attribute>

	<optional>

	  <element name="serial-element">

	    <text/>

	  </element>

	</optional>

      </group>

    </choice>

</element>