I have an element <node> which has a certain set of attributes and child
nodes that will always be there.  However, it has an additional <list>
child element, that may or may not be present with varying child
elements so <node> could have variations like this...

Given these possible variations and based on some processing elsewhere
in the document, I would like to make sure that <node> always has
<item>B</item> in its list...possibly adding <list> if it is not
present.

 <xsl:template match="node">
   <xsl:copy-of select="@*/>
   <xsl:choose>
     <xsl:when test="list/item[.='B']">
       <!-- ok so far; copy it -->
       <xsl:copy-of select="list"/>
     </xsl:when>
     <xsl:when test="list">
       <!-- list exists, but no B item; add it -->
       <list>
         <xsl:copy-of select="list/node()"/>
         <item>B</item>
       </list>
     </xsl:when>
     <xsl:otherwise>
       <!-- no list; add it -->
       <list>
         <item>B</item>
       </list>
     </xsl:otherwise>
   </xsl:choose>
   <xsl:apply-templates/>
 </xsl:template>