A union inside a simpleType fails

[Mammux]

The code does not handle a xs:simpleType that contains a union, even though each alternative in the union has a restriction.

No restriction for this simple type SimpleType { name: "", restriction: None, list: None, union: Some(Union { member_types: "" }) }

<xs:simpleType>
    <xs:union>
        <xs:simpleType>
            <xs:restriction base="xs:nonNegativeInteger">
                <xs:maxInclusive value="999"/>
            </xs:restriction>
        </xs:simpleType>
        <xs:simpleType>
            <xs:restriction base="xs:string">
                <xs:enumeration value="TBN"/>
            </xs:restriction>
        </xs:simpleType>
    </xs:union>
</xs:simpleType>