When boolean logic has a maybe value

When doing Word-to-Blogger conversion, I wanted to identify a group of code paragraphs and convert them into a single PRE element (Word-to-MediaWiki is simpler, as you just prepend a single space to the text and MediaWiki merges all code lines together). The original idea was pretty simple:

• Match w:p elements with w:pPr/w:style/@w:val = 'code';
• Check if the previous w:p element also has code style, if not, emit the PRE element and handle the whole group of code paragraphs.

The initial code worked ...

<xsl:template match="w:p[w:pPr/w:pStyle/@w:val = 'code']">
  <xsl:if test="preceding-sibling::w:p[1]/w:pPr/w:pStyle/@w:val != 'code'">
    <pre class='{w:pPr/w:pStyle/@w:val}'>
      <xsl:apply-templates select='.' mode="pre" />
    </pre>
  </xsl:if>
</xsl:template>

... but only until I've decided to add a border around the code style in Word. The border creates a wx:pBdrGroup in WordProcessingML ...

... and the xsl:if test fails if there is no preceding w:p element. To make the code work, I had to reverse the test condition and add a not at the beginning:

<xsl:template match="w:p[w:pPr/w:pStyle/@w:val = 'code']">
  <xsl:if test="not(preceding-sibling::w:p[1]/w:pPr/w:pStyle/@w:val = 'code')">
  ... rest of code ...
  </xsl:if>
</xsl:template>