xs:choice question

Hello!

I have the following xml fragment example:

<data>
 <a>a test</a>
 <b>b test</a>
<data>

the schema should check the following rules (three options):
- element a must exist, b may not
- element b must exist, a may not
- element a and b must exist

Therefor I have defined the following choice:
...
<xs:choice>
 <xs:element name="a" type="xs:string"/>
 <xs:element name="b" type="xs:string"/>
 <xs:sequence>
   <xs:element name="a" type="xs:string"/>
   <xs:element name="b" type="xs:string"/>
 </xs:sequence>
</xs:choice>
...

The xml editor raises an error - something about non determenistic declaration...

Do you have a suggestion how to define these three rules?

Thanks for your help!
Markus
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