XSD context-dependent types; wrapper types - validation

Question

I am writing a schema for XML documents like this:
<workbook>
<worksheet>
<column/>
</worksheet>
</workbook>
Where each node can be wrapped into the env element (e.g.)
<workbook>
<env>
<worksheet>
<column/>
</worksheet>
</env>
</workbook>
or
<workbook>
<worksheet>
<env>
<column/>
</env>
</worksheet>
</workbook>
So, depending on the location of the env element, it can have different children (env-child of workbook must have worksheet children and env-child of worksheet must have column children)
When I am declaring env elements as nested, I get multiply-defined element errors.
How could I write xsd for such document, or is it at all possible?
Thanks in advance!

Answer 1

Yes, it is possible, because you can have elements with the same name env but with different types (structure) in different places - something linke this:
<xs:element name="workbook">
<xs:complexType>
<xs:choice>
<xs:element name="env">
<xs:complexType>
<xs:sequence>
<xs:element ref="worksheet"/>
</xs:sequence>
</xs:complexType>
</xs:element>
<xs:element ref="worksheet"/>
</xs:choice>
</xs:complexType>
</xs:element>
<xs:element name="worksheet">
<xs:complexType>
<xs:choice>
<xs:element name="env">
<xs:complexType>
<xs:sequence>
<xs:element ref="column"/>
</xs:sequence>
</xs:complexType>
</xs:element>
<xs:element ref="column"/>
</xs:choice>
</xs:complexType>
</xs:element>
<xs:element name="column" type="xs:string">
</xs:element>