identity constraints and substitution groups

Do the xpath expressions in an identity constraint have to match the actual
name of an element or is it sufficient to match a substitution group?

I.e.  suppose

<xs:element name='column'/>
<xs:element name='specialColumn' substitutionGroup='column'/>

<xs:key name='kColumn'>
    <xs:selector xpath='column'/>
    <!-- etc -->
</xs:key>

Will the selector match 'specialColumn' elements as well as 'column'
elements?

Regards,
Mark Thornton