XSD to allow ul and ol tags inside li tag xsd

Question

I have written a rule for li tag as below and i need to modify this to allow ul or ol tags inside li tag and i don't want to make it mandatory that it should contain ol or ul inside li tag. It should be optional.

<xs:attributeGroup name="liAttrs">
        <xs:attribute name="id" type="objectId"/>
    </xs:attributeGroup>
<xs:element name="li">
        <xs:annotation>
            <xs:documentation>
                list item
            </xs:documentation>
        </xs:annotation>
        <xs:complexType mixed="true">
            <xs:complexContent>
                <xs:extension base="Flow">
                    <xs:attributeGroup ref="liAttrs"/>
                </xs:extension>
            </xs:complexContent>
        </xs:complexType>
    </xs:element>

New into xsd. Thanks in advance.