Basically, I want a template that will output the text of a node for an
  arbitrary XPath if that node exists.  If the node doesn't exist, I want
  it to produce "--".


in xslt 2

<xsl:value-of select="(/path/to/single/node,'--')[1]"/>

in xslt 1

<xsl:variable name="n" select="/path/to/single/node"/>
<xsl:value-of select="$n"/>
<xsl:if test="not($n)">--</xsl:if>


David

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