How to override a Xml Schema element?

Question

In the following sample:

• A employeeID always is a personID, but not every personID is a employeeeID.
• employeeID is a restriction of personID.
• A Empolyee always is a Person, but not every Person is a Employee.
• Employee is a extension from Person.
• A employeeID must be validated as "employeeID", but also, must be acessible as a "personID".

The following code generates the following validation error in the ID declaration of the Employee type: "Elements with the same name and in the same scope must have the same type."

Help!

 

 

	<xs:simpleType name="PersonID">
		<xs:restriction base="xs:token"></xs:restriction>
	</xs:simpleType>

	<xs:simpleType name="EmployeeID">
			<xs:restriction base="PersonID"></xs:restriction>
	</xs:simpleType>

	<xs:complexType name="Person">
		<xs:sequence>
			<xs:element name="ID"
						type ="PersonID"></xs:element>
		</xs:sequence>
	</xs:complexType>

	<xs:complexType name="Employee">
		<xs:complexContent>
			<xs:extension base="Person">
				<xs:sequence>
					<xs:element name="ID"
								type="EmployeeID"
								></xs:element>
				</xs:sequence>
			</xs:extension>
		</xs:complexContent>
	</xs:complexType>