Unexpected Ambiguous Model in XSD Schema

[Eliot Kimber <ekimber@contrext.com>]

I'm working with a schema developed by others. Both Xerces and Saxon 9.7, run from within Oxygen, report ambiguity on this content model:

<xs:element name="figure">
  <xs:complexType>
   <xs:choice>
     <xs:sequence>
       <xs:element ref="figcaption" minOccurs="1" maxOccurs="1"/>
       <xs:choice maxOccurs="unbounded">
         <xs:element ref="img"/>
       </xs:choice>
    </xs:sequence>
   <xs:sequence>
     <xs:choice maxOccurs="unbounded">  
       <xs:element ref="img"/>
     </xs:choice>
     <xs:element ref="figcaption" minOccurs="1" maxOccurs="1"/>	
  </xs:sequence>
  <xs:sequence>
   <xs:choice maxOccurs="unbounded">
     <xs:element ref="img"/>
   </xs:choice>
  </xs:sequence>
</xs:choice>
</xs:complexType>
</xs:element>

The error is that element "img" is allowed in multiple places.

If I was coding this as a DTD it would be:

<!ELEMENT figure
   ((figcaption, (img)+) |
    ((img)+, figcaption) |
    ((img)+))
>

Which is definitely not ambiguous.

The intent here is to allow this instance:

<root>
  <figure>
    <figcaption></figcaption>
    <img></img>
  </figure>
  <figure>
    <figcaption></figcaption>
    <img></img>
  </figure>
  <figure>
    <img></img>
  </figure>
  
</root>

But not:

<figure>
  <figcaption/>
  <img/>
  <figcaption/>
</figure>

So my question: Why is the XSD model ambiguous when the choice does not repeat and therefore no particle can occur more than once?

How does one implement this type of content model in XSD?

Thanks,

Eliot
--
Eliot Kimber
http://contrext.com