templates with optional paramters

I have an xslt template with 2 paramter. One of them must not be specified.
How can I find out if one or two params where specified? Do I need two versions of the template with mode? Or can I check with =''?

<xsl:template name="foo">
  one mandatory paramter
  one optional paramter
...do things...
</xsl:template>

One can call it by:

<xsl:call-template name="foo">
  <xsl:with-param name=="bar_mandatory" select="$whatever" />
  <xsl:with-param name="bar_optional" select="$whatever" />
</xsl:call-template>

OR

<xsl:call-template name="foo">
  <xsl:with-param name=="bar_mandatory" select="$whatever" />
</xsl:call-template>


Rob
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