Xml schema having key for different depth

I have a flexible configuration that uses a <directory>-element at different hierarchical levels. Now I would like to ensure that one directory can only be used once.

<xs:element name="rootElement">

    <xs:complexType>
        <xs:sequence>
            ...
        </xs:sequence>
        <xs:attribute name="xyz" use="required" type="xs:integer"/>
    </xs:complexType>

    <xs:key name="key_directory">
        <xs:selector xpath="VARIABLE_DEPTH???/directory"/>
        <xs:field xpath="@path"/&g开发者_Go百科t;
    </xs:key>

</xs:element>

Can this be done like this?

You can use .//directory (padding:................ the forum doesn't like short answers ........)