Proper "Next" and "Previous" links when chunking using Docbook <set>'s.

[raducoravu]

Based on this post:

https://lists.oasis-open.org/archives/docbook-apps/201706/msg00001.html

On this Docbook 5 example:

        <?xml version="1.0" encoding="UTF-8"?>
        <?xml-model href="http://docbook.org/xml/5.1/rng/docbookxi.rng" schematypens="http://relaxng.org/ns/structure/1.0"?>
        <?xml-model href="http://docbook.org/xml/5.1/sch/docbook.sch" type="application/xml" schematypens="http://purl.oclc.org/dsdl/schematron"?>
        <set xmlns="http://docbook.org/ns/docbook" xmlns:xi="http://www.w3.org/2001/XInclude"
            xmlns:xlink="http://www.w3.org/1999/xlink" version="5.1">
            <info>
                <title>Set A</title>
                <abstract>
                    <title>First set</title>
                    <para>This is a set of two articles, one of which is in another set</para>
                </abstract>
            </info>
            
            <article>
                <title>Here is an article title1</title>
                <info>
                    <abstract>
                        <title>Title of an article abstract</title>
                        <para>Text of abstract</para>
                    </abstract>
                </info>
                <simplesect>
                    
                    <title>Title of simple section</title>
                    <para>Text of simple section</para>
                </simplesect>
            </article>
            
            
            <set>
                <info>
                    <title>Set B containing one article in a set</title>
                    <abstract>
                        <title>Second set</title>
                        <para>Is this text visible?</para>
                    </abstract>
                </info>
                
                <article>
                    <title>Here is an article title2</title>
                    <info>
                        <abstract>
                            <title>Title of an article abstract</title>
                            <para>This article is in another set. Did you see the abstract of set B?</para>
                        </abstract>
                    </info>
                    <simplesect>
                        
                        <title>Title of simple section</title>
                        <para>Text of simple section</para>
                    </simplesect>
                </article>
                
            </set>
            
        </set>

when chunking to HTML, for the first article chunk, the "Next" link should probably point to the Set which follows it.

[raducoravu]

👍