using document in xsl:key match...

I am using the libxslt from the GNOME project (http://xmlsoft.org/XSLT/).
I would like to confirm that this is a bug, and not a design issue.

I am trying to use xsl:key to index a set of elements from another document.
When I try to do:
<xsl:key match="document('ed.xml')/root/sup" use="@name"/>
I get:
XPath error : Invalid expression
//document('ed.xml')/root/sup
           ^
compilation error: file tb.xml line 7 element key
xsl:key : XPath pattern compilation failed '//document('ed.xml')/root/sup'

It appears that a // is being prepended to my XPath query, which is causing
it to fail.  Is this suppose to be a valid construct (using document in
an xsl:key match term)?  Or is this a bug in the library?

After additional reading of: http://www.w3.org/TR/xslt#NT-Pattern
It appears this construct is not allowed.  Is this correct?

-- 
John-Mark Gurney
nCircle Network Security, Inc.