union of complex types

Hi all,
    XSD language already provides, for union syntax for the simple types as
follows,

<!-- here the XSD language already provides, that Type1 and Type2 can only
be simple types -->
<xs:simpleType name="ST1">
    <xs:union memberTypes="Type1 Type2" />
</xs:simpleType>

I propose, a similar new facility for XSD complex types, as follows,

<!-- Type3 and Type4 can be any complex types -->
<xs:element name="X">
     <xs:complexType>
         <xs:union memberTypes="Type3 Type4" />
     </xs:complexType>
</xs:element>

Any thoughts shall be interesting to know.


-- 
Regards,
Mukul Gandhi