Hi Roger,

    How about following approach.

The XML instance document is like following,

<?xml version="1.0"?>
<Publication>
   <Title>abc</Title>
   <ISBN>123-456-789</ISBN>
</Publication>

And with the following XML Schema 1.1 document,

<?xml version="1.0"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
 
  <xs:element name="Publication">
     <xs:complexType>
       <xs:all>
          <xs:element name="Title" type="xs:string"/>
          <xs:any processContents="skip" minOccurs="0"/>
       </xs:all>
       <xs:assert test="if (*[not(self::Title)]) then *[not(self::Title)][count(node()) = 1 and text()] else true()"/>
       <xs:assert test="if (*[not(self::Title)]) then not(*[not(self::Title)]/@*) else true()"/>
       <xs:assert test="if (*[not(self::Title)]) then (*[not(self::Title)]/name() = ('ISBN','Edition')) else true()"/>
     </xs:complexType>
  </xs:element>
 
</xs:schema>

I meant following with the above cited schema document,

The Title element can have an optional sibling element, and its name be either ISBN or Edition. That sibling element, cannot have non text child, and it cannot have attributes as well.

I guess, above schema document is not a great alternative, to solutions already proposed. But just another solution :)