Best way to ensure one element out of a set

> Hi,
> 
> is this the best way to ensure that one element out of a set is used? 
> I have the following xml:
> 
> --- test.xml begin ---
> <?xml version="1.0" encoding="ISO-8859-1"?>
> <top xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
>      xsi:noNamespaceSchemaLocation="test.xsd">
>      
>      <conditions>
> 
>          <primitive_condition_0 />
>          <primitive_condition_1 />
>          <primitive_condition_2 />    
> 
>          <complex_condition />
>      </conditions>
> </top>
> --- test.xml end ---
> 
> I want to make sure that at least one of the primitive_condition elements ist used, but I don't care which one.
> I have come up with the following solution (which works) but are now wondering if this is the best way to do this. 
> My qestion is: Is there a more straight forward way to do this than to have a group of choices?
> 
> --- test.xsd begin ---
> <?xml version="1.0" encoding="UTF-8"?>
> <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
>     <xs:element name="top" >
>         <xs:complexType>
>             <xs:sequence>
>                 <xs:element name="conditions" type="conditions" />
>             </xs:sequence>
>         </xs:complexType>
>     </xs:element>
>     
>     <xs:complexType name="conditions" >
>         <xs:sequence>
>             <xs:group ref="primitive_conditon" minOccurs="0" maxOccurs="unbounded" />
>             <xs:element name="complex_condition" type="complex_condition" minOccurs="0" maxOccurs="unbounded" />
>         </xs:sequence>
>     </xs:complexType>
>     
>     <xs:group name="primitive_conditon" >
>         <xs:choice>
>             <xs:element name="primitive_condition_0" />
>             <xs:element name="primitive_condition_1" />
>             <xs:element name="primitive_condition_2" />
>         </xs:choice>
>     </xs:group>
>     
>     <xs:complexType name="complex_condition" />
> </xs:schema>
> --- test.xsd end ---
> 
> Thanks for taking an interest.
> 
> Erwin
>