<xs:complexType name="any_tag">
      <xs:sequence>
        <xs:any minOccurs="0" maxOccurs="unbounded" processContents="lax"/>
      </xs:sequence>
    </xs:complexType>
    <xs:element name="someelement">
      <xs:complexType>
        <xs:complexContent>
          <xs:extension base="any_tag">
            <xs:attribute name="someattr" type="xs:string"/>
          </xs:extension>
        </xs:complexContent>
      </xs:complexType>
    </xs:element>