How to refer to xpath using variables?

Hi.

I'm bit puzzled. The following example does not work with mozilla.

XML-file:
...
<include-data xpath="/foo/bar" mode="foo" />
...

XSL-file:
...
<xsl:template match="include-data">
  <xsl:variable name="xpath"><xsl:value-of
select="xpath"></xsl:variable>
  <xsl:variable name="mode="><xsl:value-of
select="mode"></xsl:variable>
  <xsl:apply-templates select="$xpath" mode="$mode" />
</xsl:template>
...

The error message I get is: "Error loading stylesheet: (null)"

However, if I remove the mode-attribute, the error message is: "Error
during XSLT transformation: An XPath expression was expected to return
a NodeSet."

What to do?