xsd inheritance

i have an xsd

<xsd:complextype name="a">  
        <xsd:complexcontent>  
            <xsd:sequence>  
                <xsd:element name="options">  
                    <xsd:complextype>  
                        <xsd:sequence>  
                            <xsd:element name="day">  
                            ...  
                            </xsd:element>  
                        </xsd:sequence>  
                    </xsd:complextype>  
                </xsd:element>  
            </xsd:sequence>  
        </xsd:complexcontent>  
</xsd:complextype>  

<xsd:complextype name="b">  
    <xsd:complexcontent>
        <xsd:extension base="a">
        ...what here...
        </xsd:extension>
    </xsd:complexcontent>
</xsd:complextype>  

so fundamentally i wish category have method options (day, week example) following i wish b get have a's options an additional 2 3 options hours, seconds.