However, I'm not sure whether this is the right design for your problem. I
would be inclined to stick with the traditional approach: the
special-purpose stylesheet module (here, the XHTML output module) should
import the general-purpose module, not the other way around; and you then
decide which special-purpose module to use by using that as your entry point
to the transformation.

	<xsl:variable name="citerefs" select="//db:biblioref/@linkend |
									//cite:biblioref/@cite:key |
									//tei:cit/@href"/>