restriction against base type (choice - elt)

Hi..
 
According to specification the following schema is invalid since according
to the table in 
http://www.w3.org/TR/xmlschema-1/#cos-particle-restrict 
it is given that the case "choice (Derived particle) to element(Base
Particle) is Forbidden"
 
 
 
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
 
<xs:element name="head"/>
<xs:element name="m1" substitutionGroup="head"/>
<xs:element name="m2" substitutionGroup="head"/>
 
<xs:complexType name="base">
 <xs:sequence>
  <xs:element ref="head"/>
 </xs:sequence>
</xs:complexType>
 
<xs:complexType name="derived">
 <xs:complexContent>
  <xs:restriction base="base">
   <xs:sequence>
          <xs:choice>
             <xs:element ref="m1"/>
             <xs:element ref="m2"/>
          </xs:choice>
   </xs:sequence>
  </xs:restriction>
 </xs:complexContent>
</xs:complexType>
 
</xs:schema>

 
Can anybody please tell me whether the above schema is valid or invalid
against the schema spec.
 
Thanks and Regards,
Bharath.