Accessing the main document from a document()

Hi,

I am trying to write an XSL stylesheet that displays the data of the
document it is attached to under the control of a second document.
This second document describes the formatting and data types of the
data in the main document.  It also defines the order in which the
fields from the main document are displayed.

Main document:

<?xml-stylesheet type="text/xsl" href="stylesheet.xsl" ?>
<data device-type="foo">
 <value name="a">blah</value>
 <value name="b">blub</value>
</data>

Formatting document:

<definitions>
 <device type="foo">
  <parameter name="b">... formatting instructions for field b..</parameter>
  <parameter name="a">... formatting instructions for field a...</parameter>
 </device>
 <device ... more devices>
 </device>
</definitions>