Test expression : Is Context Node an ancestor of node

Hi guys, 

Gee I must have been annoying this week (apologies).
I am coming to grips with navigating through xml documents using xpath,
using axis' ect.

Question:

I want to use a test expression to test whether the node currently matched
(context node?) is an ancestor of a node with a specified id.

Logic Process:
when currentNode is an ancestor of the node with id = 9.
render styles within xsl:when.

Below is my attempt, however this only generates an error.

What mondifications do I need to accomplish this task.

     <xsl:choose>
        <xsl:when test="@id=$id | ancestor::tree_node[@id=$id]">
        </xsl:when>
        <xsl:otherwise>
        </xsl:otherwise>
  </xsl:choose>

Cheers, 
Adam