[xsl] How to select child elements of a given type

[Gustave Stresen-Reuter <tedmasterweb@xxxxxxx>]

    <xsl:variable name="all">
    	<xsl:copy-of select="$path2pageAsSource | /root/folder" />
    </xsl:variable>

        <xsl:for-each select="$all/descendant-or-self::folder">
            stuff...
        </xsl:for-each>

This for-each construct works with Saxon but not with Sablotron, 
libxslt, or Xalan-J.

If I remove the /descendant-or-self::folder it works fine (but of 
course, doesn't select the nodes I'm looking for).

I've tried using other shortcuts ($all//*/folder) and such, but they 
all return the same error.

Any ideas what the problem might be?

Ted Stresen-Reuter