Cross-references to indexes

Capture cross-references to Index headings by capturing the id of the indexGroup or indexList that contains the heading in the ref attribute of the the xref.

Example with indexGroup target

If we have


      <indexGroup id="med-9780199228881-indexGroup-1">
        <indexList id="med-9780199228881-indexList-1" indexListType="regular"><titleGroup>
 <title>
   <p>Index</p>
 </title></titleGroup><alphabet>A</alphabet><indexItem1 id="med-9780199228881-indexItem1-1">
                     <iHeadword>α-fetoprotein</iHeadword>
 <xrefGrp>
                         <pageNum pageId="10">
                             <xref ref="med-9780199228881-chapter-1-milestone-10">10</xref>
                         </pageNum>–<pageNum pageId="12">
                             <xref ref="med-9780199228881-chapter-1-milestone-12">12</xref>
                         </pageNum>, <pageNum pageId="140">
                             <xref ref="med-9780199228881-chapter-7-milestone-140">140</xref>
                         </pageNum>
                     </xrefGrp>
                 </indexItem1><!-- Content skipped for brevity -->
        </indexList>
      </indexGroup>

Then an xref to Index has ref="med-9780199228881-indexGroup-1"

Example with indexList and indexGroup targets

If we have a more complex index, like in Practical Drug Therapy, eg,


      <indexGroup id="med-9780198530077-indexGroup-1">
        <titleGroup><title>
 <p>Indexes</p></title>
        </titleGroup>
        <indexList id="med-9780198530077-indexList-1" indexListType="regular"><titleGroup>
 <title>
   <p>Drug Index</p>
 </title></titleGroup><alphabet>A</alphabet><indexItem1>
 <!-- Content skipped for brevity --></indexItem1>
        </indexList>
        <indexList id="med-9780198530077-indexList-2" indexListType="regular"><titleGroup>
 <title>
   <p>Subject Index</p>
 </title></titleGroup><alphabet>A</alphabet><indexItem1>
 <!-- Content skipped for brevity --></indexItem1>
        </indexList>
        <!-- Content skipped for brevity -->
      </indexGroup>

Then:

an xref to Drug Index has ref="med-9780198530077-indexList-1"
an xref to Subject Index has ref="med-9780198530077-indexList-2"
an xref to Indexes has ref="med-9780198530077-indexGroup-1"