[xsl] fo basic link question

["Mark Wilson" <mark@xxxxxxxxxxxx>]

Every example for building a uri("...") I can find always fills the 
parentheses with a quoted string, like uri("s.pdf"). I want to use the value 
of an attribute:

<fo:basic-link external-destination="url(Heading/@file)" color="blue">

Can I do that? If so, how?

Thanks,
Mark