Atleast one element should be present in ComplexType.

Hi,
	I have a complexType with two element children and I need to make
sure that atleast one of that should be present.
Here is the sample.

	<xs:element name="REQUEST">
		<xs:complexType>
			<xs:sequence>
				<xs:element ref="OPTION" minOccurs="0"
maxOccurs="unbounded"/>
				<xs:element ref="DATA" minOccurs="0"/>
			</xs:sequence>
		</xs:complexType>
	</xs:element>

In the above example, I need to make sure that atleast on among OPTION, DATA
should be present.

Thanks,
Raj.