XSD 1.1 assertions

I am using the XSD 1.1 and trying to add assertions to the complex types.

 I have an abstract complex type and another concrete complex type
extending the abstract complex type.

 For example:

 <xs:complexType name="abstract-comp-type" abstract="true">

                                <xs:sequence>

                                                <xs:element
type="xs:string" name="description" minOccurs="1" maxOccurs="1" />

                                                <xs:element name="comp-id"
type="xs:ID" minOccurs="1" />

                </xs:complexType>



                <xs:complexType name="concrete-comp-type">



                                <xs:complexContent>

                                                <xs:extension
base="abstract-comp-type">


<xs:sequence>


<xs:element name="element1" type="xs:string" minOccurs="1" fixed="test"/>


</xs:sequence>

                                                </xs:extension>

                                </xs:complexContent>

                </xs:complexType>

 If I have more than one complex types extending from abstract-comp-type,
how do I add validation to check if all the sub types have an element named
element1?

 The reason why I need to add element1 in the sub type is to provide a
fixed value. I cannot use restriction because the sub type will have more
elements than the abstract type.