> But that's not particularly pretty and doesn't scale very 
> well.  In that case it's often best to define a looser 
> schema, and then impose the constraint at the application 
> level.  An example schema for that might be:
> 
>     <xs:complexType name='..'>
>             <xs:sequence>
>                 <xs:element name='element1' minOccurs='0'../>
>                 <xs:element name='element2' minOccurs='0'.../>
>                 <xs:element name='element3' minOccurs='0'../>
>                 <xs:element name='element4' minOccurs='0'../>
>             </xs:sequence>
>     <xs:complexType>


Or with XML Schema 1.1 you can use assertions (available in Saxon 9.0, just
released). For example the above type definition could become:

     <xs:complexType name='..'>
             <xs:sequence>
                 <xs:element name='element1' minOccurs='0'../>
                 <xs:element name='element2' minOccurs='0'.../>
                 <xs:element name='element3' minOccurs='0'../>
                 <xs:element name='element4' minOccurs='0'../>
             </xs:sequence>
             <xs:assert test="count(*) = 1 to 4"/>
     <xs:complexType>

Michael Kay
http://www.saxonica.com/