I am taking my first steps in XML-schema land, using XSD, and I'm trying to figure out how to add a constraint such that an attribute (defined with a fixed value) may only be present on one element in an unbounded set of elements.

Here's an example

```
<Object>
    <Element>Value1</Element>
    <Element>Value2</Element>
    <Element Default="true">Value3</Element>
    <Element>Value4</Element>
    <Element>Value5</Element>
</Object>

```

I'd like to constrain the `Default` attribute such that it cannot appear on more than one `Element`.

Here's the XSD schema for the above:

```
<xs:element name="Object">
    <xs:complexType>
        <xs:element name="Element" maxOccurs="unbounded">
            <xs:simpleContent>
                <xs:extension base="xs:string">
                    <xs:attribute name="Default" fixed="true"/>
                </xs:extension>
            </xs:simpleContent>
        </xs:element>
    </xs:complexType>
</xs:element>

```

I've tried messing with `xs:unique`, `xs:selector` and `xs:field`, but reached no meaningful results, not sure if that's the correct approach.

Thanks!