add indexlist tag in DITAMAP

[WanEye]

Add <indexlist> condtionally so that DITAMAP reads

content DITAMAP

    <backmatter>
        <booklists>
            <indexlist/>
        </booklists>
    </backmatter>
</bookmap> 

pseudocode to add the right tags

If exists <backmatter> then:
    if exists <booklist> then:
        if exists <indexlist>
           # do nothing
       else: write (<indexslist/>)
    else:
         write(<booklist> ...</booklist>)
else:
    write(<backmatter> ... </backmatter>)
endif