Susan,

I hope you and your folks are all right after the storms
You're welcome, here's one more modification you can make for a slight optimization:
inside <xsl:for-each select="exsl:node-set($output)/*">:

     <xsl:variable name="title">
      <xsl:if test="not(preceding-sibling::*[@doc-number = current()/@doc-number])">
       <xsl:value-of select="@title"/>
      </xsl:if>
     </xsl:variable>

Instead of testing all the preceding siblings, it suffices to test only the previous sibling:

      <xsl:if test="not(preceding-sibling::*[1]/@doc-number = @doc-number)">

(well, with large input files it might be more than slight)

Greetings
Anton Triest