xsi:type and unions

Is there any text in the W3C XML Schema recommendation that makes the
following instance invalid according to the schema below. 

<s xmlns="b64" xmlns:xs="http://www.w3.org/2001/XMLSchema"; 
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance";
xsi:type="xs:hexBinary">1234</s>


<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema";
targetNamespace="b64" 
xmlns:b64="b64"> 
        <xs:element name="s" type="b64:myType2"/> 

        <xs:simpleType name="myType"> 
                <xs:union memberTypes="xs:base64Binary xs:hexBinary
xs:NOTATION"/> 
        </xs:simpleType> 

        <xs:simpleType name="myType2"> 
                <xs:restriction base="b64:myType"> 
                        <xs:pattern value="abcd"/> 
                </xs:restriction> 
        </xs:simpleType> 
</xs:schema>

-- 
PITHY WORDS OF WISDOM 
To succeed in politics, it is often necessary to rise above your
principles.                                                         

This posting is provided "AS IS" with no warranties, and confers no
rights.