XPath preceding depth-first tree element - xpath

Question

I have a tree structure:
<pages>
<page id="1">
<page id="1A" />
<page id="1B" />
</page>
<page id="2" />
<page id="3">
<page id="3A" />
<page id="3B" />
<page id="3C" />
</page>
</pages>
I want to emit "previous" links with XSLT. I have succeeded in creating "next" links.
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">
<xsl:output method="xml" indent="yes"/>
<xsl:template match="/pages">
<pages>
<xsl:for-each select=".//page">
<page id="{#id}" prev="???" next="{(child::page|following::page)/#id}" />
</xsl:for-each>
</pages>
</xsl:template>
</xsl:stylesheet>
Expected output:
$ xsltproc test.xsl test.xml
<?xml version="1.0"?>
<pages>
<page id="1" prev="" next="1A"/>
<page id="1A" prev="1" next="1B"/>
<page id="1B" prev="1A" next="2"/>
<page id="2" prev="1B" next="3"/>
<page id="3" prev="2" next="3A"/>
<page id="3A" prev="3" next="3B"/>
<page id="3B" prev="3A" next="3C"/>
<page id="3C" prev="3B" next=""/>
</pages>

Answer 1

I think
<page id="{#id}" prev="{(ancestor::page[1]/#id|preceding::page[1]/#id)[last()]}" next="{(child::page|following::page)/#id}" />
is what you are looking for.

Answer 2

Another way you could look at it:
XSLT 1.0 (with a node-set() extension function)
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:exsl="http://exslt.org/common"
extension-element-prefixes="exsl">
<xsl:output method="xml" version="1.0" encoding="UTF-8" indent="yes"/>
<xsl:template match="/pages">
<xsl:variable name="all-pages">
<xsl:copy-of select=".//page"/>
</xsl:variable>
<pages>
<xsl:for-each select="exsl:node-set($all-pages)/page">
<page id="{#id}" prev="{preceding-sibling::page[1]/#id}" next="{following-sibling::page[1]/#id}" />
</xsl:for-each>
</pages>
</xsl:template>
</xsl:stylesheet>