not(not() and not())

I seem to have a problem with logic. Can someone explain why this XPath
expression:

<!-- Paint border unless it's the last row in the last rowgroup. -->
<xsl:if test="not(not(parent::tr/following-sibling::tr) and
              not(ancestor::tbody/following-sibling::tbody))">
  <xsl:call-template name="draw-border-bottom"/>
</xsl:if>

is not exactly the same as this:

<!-- Paint border unless it's the last row in the last rowgroup. -->
<xsl:if test="parent::tr/following-sibling::tr and
              ancestor::tbody/following-sibling::tbody">
  <xsl:call-template name="draw-border-bottom"/>
</xsl:if>

The first gives the correct result, but the second paints borders only on
all but the last rowgroup. Shouldn't "... and ..." always be the same as
"not(not(...) and not(...))"? If not, is it a better way to write the above?

Gustaf


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