Modifying unselected branches in xslt

I want to modify a node in a branch depending on it's sibling. In the following example, I want to add <var2 Value='D'/> to the pre-existing <nest2> if nest1 contains <var1 Value='A'>.

Input:

<variables>
  <nest1>
    <var1 Value='A'/>
    <var1 Value='B'/>
  </nest1>
  <nest2>
    <var2 Value='C'/>
  </nest2>
</variables>

Output

<variables>
  <nest1>
    <var1 Value='A'/>
    <var1 Value='B'/>
  </nest1>
  <nest2>
    <var2 Value='C'/>
    <var2 Value='D'/>
  </nest2>
</variables>

I can match <var1 Value='A'/>, but I don't know how to modify the pre-existing <nest2>. I can match <nest2>, but I don't know how to check to see if <var1 Value='A'/> exists in <nest1>. I'm ju开发者_JAVA技巧st stumped on where to go from here.

I am using xslt 1.0.

Here is a sample stylesheet that should do the job:

<xsl:stylesheet
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  version="1.0">

  <xsl:strip-space elements="*"/>
  <xsl:output indent="yes"/>

  <xsl:template match="@* | node()">
    <xsl:copy>
      <xsl:apply-templates select="@* | node()"/>
    </xsl:copy>
  </xsl:template>

  <xsl:template match="nest2[preceding-sibling::nest1/var1[@Value = 'A']]">
    <xsl:copy>
      <xsl:apply-templates select="@* | node()"/>
      <var2 Value="D"/>
    </xsl:copy>
  </xsl:template>

</xsl:stylesheet>