XML Schema question: A <xs:choice> equivalent for attributes?

["Spoonauer, Michael" <Michael.Spoonauer@FMR.COM>]

I wish to create an XML document in which one of its elements must be
defined with one of two known attributes, but not both.
 
For example, I would like to define a "socketHandler" element as follows:
 
<xs:element name="socketHandler">

    <xs:complexType>

        <xs:attribute name="hostName" type="xs:string"/>

        <xs:attribute name="hostAddress" type="xs:string"/>

        <xs:attribute name="port" type="xs:int" use="required"/>

    </xs:complexType>

</xs:element>


but I only want a user to be able to define either a hostName attribute
(e.g., hostName="<servername>") or a hostAddress (e.g.,
hostAddress="<IP_address>") attribute, but not both.  It appears that the
<xs:choice> construct accomplishes this for elements.  Is there a functional
equivalent for attributes, or is there a better way to approach this?

If there's a more appropriate forum for this type of question, just let me
know.
 
Thanks in advance,
Mike
 

Michael V. Spoonauer
Fidelity Investments
E-mail: Michael.Spoonauer@fmr.com