XSD Question

Hi Everyone!

Can you tell me how to make this field required using this XSD?

<?xml version=3D"1.0" encoding=3D"UTF-8" standalone=3D"yes"?>
<xs:schema xmlns:xs=3D"http://www.w3.org/2001/XMLSchema"
elementFormDefault=3D"qualified">
	<xs:element name=3D"assignedto">
		<xs:simpleType>
			<xs:restriction base=3D"xs:string"/>
		</xs:simpleType>
	</xs:element>
	<xs:element name=3D"request">
		<xs:complexType>
			<xs:choice maxOccurs=3D"unbounded">
				<xs:element ref=3D"assignedto"/>

			</xs:choice>
		</xs:complexType>
	</xs:element>
	<xs:element name=3D"requests">
		<xs:complexType>
			<xs:sequence>
				<xs:element ref=3D"request"
maxOccurs=3D"unbounded"/>
			</xs:sequence>=09
		</xs:complexType>
	</xs:element>
</xs:schema>

Thanks!
James