using multiple namespaces in one element

Hello list users,

I have an element which should contain elements from MathML. The problem 
is when I include an MathML example into the instance document the 
parser says that the prefix mml is not bounded. MathML should only be 
possible inside of the mathphrase element.

    <xs:element name="mathphrase">
        <xs:complexType>
            <xs:choice>
                <xs:any namespace="http://www.w3.org/1998/Math/MathML" 
minOccurs="0" maxOccurs="unbounded"/>
            </xs:choice>
        </xs:complexType>
    </xs:element>

How can I include the whole MathML namespace into my schema?

I am thankful for every hint,

Andreas