Selecting from a node-set using variable path

For the life of me, I don't know how to do this even
though it seems like it would be very easy.

I have a variable, myNodeSet, which points to a node set,
in this case a set of 1 document element node.
I have another variable, myPath, which reflects a
(relative) location path to nodes I want to select from
myNodeSet.

This works:
 <xsl:variable name="t1" select="$myNodeSet//a/b/c"/>

This doesn't:
 <xsl:variable name="t2" select="$myNodeSet/$myPath"/>

I'm using MSXML 4.0 and it's complaining with:
 NodeTest expected here.
 $myNodeSet/-->$<--myPath
which makes sense.

How do I filter down my node-set based upon some variable
location path?

Thanks
--
Gary F.
Sr. Software Engineer
Fast Track Systems, Inc