[XSD] 0:1-Restriction

[BigShane]

Hi!

My code so far:

<xs:element name="abc" minOccurs="0" maxOccurs="10">
     <xs:simpleType>
          <xs:restriction base="xs:string">
               <xs:enumeration value="a" />
               <xs:enumeration value="b" />
               <xs:enumeration value="c" />
               <xs:enumeration value="d" />
          </xs:restriction>
     </xs:simpleType>
</xs:element>

I'd like to have a restriction, which doesn't allow to use one of the above enumerations twice.

So, that should be okay:

<abc>a</abc>
<abc>b</abc>
<abc>d</abc>

This one shouldn't be:

<abc>[B]a[/B]</abc>
<abc>b</abc>
<abc>[B]a[/B]</abc>

How am I able to have such a restriction? I'm Grateful foy any help . Greeting, Thomas D.