Frank Steimke wrote:

why not

<xs:sequence>
  <xs:element name="Title" type="xs:string"/>
  <xs:choice>
    <xs:element name="ISBN" type="xs:string"/>
    <xs:element name="Edition" type="xs:string"/>
  </xs:choice>
</xs:sequence>

You are correct that for this example it is possible to factor out the declaration of the Title element. But there are examples where this is not possible, such as this:

<xs:choice>

    <xs:sequence>

        <xs:element name="ISBN" minOccurs="0" type="xs:string"/>

        <xs:element name="Title" type="xs:string"/>

    </xs:sequence>

    <xs:sequence>

        <xs:element name="Title" type="xs:string"/>

        <xs:element name="Edition" type="xs:string"/>

    </xs:sequence>

</xs:choice>

Notice that the ISBN element is optional, so we get the dreaded “Unique Particle Attribution” error.

/Roger