XSLT: combining two consecutive elements until element list is empty - xslt

Question

I have this XML
<participants>
<event>Seminar</event>
<location>City somewhere</location>
<first_name>Carl</first_name>
<last_name>Smith</last_name>
<first_name>John</first_name>
<last_name>Somebody</last_name>
<first_name>Lisa</first_name>
<last_name>Lint</last_name>
<first_name>Gabriella</first_name>
<last_name>Whowho</last_name>
</participants>
Which I would need to be transformed into this:
<participants>
<event>Seminar</event>
<location>City somewhere</location>
<persons>
<person>
<given_name>Carl</given_name>
<surname>Smith</surname>
</person>
<person>
<given_name>John</given_name>
<surname>Somebody</surname>
</person>
<person>
<given_name>Lisa</given_name>
<surname>Lint</surname>
</person>
<person>
<given_name>Gabriella</given_name>
<surname>Whowho</surname>
</person>
</persons>
</participants>
The number of persons can be any number, sometimes there might be empty elements (if both first and last names would be empty, then the person would not be created.
I have hard time getting started with this transformation.

Answer 1

If you simply process the first_name elements and use XPath navigation to select its sibling last_name you get
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
version="1.0">
<xsl:output indent="yes"/>
<xsl:template match="#* | node()">
<xsl:copy>
<xsl:apply-templates select="#* | node()"/>
</xsl:copy>
</xsl:template>
<xsl:template match="participants">
<xsl:copy>
<xsl:apply-templates select="*[not(self::first_name | self::last_name)]"/>
<xsl:apply-templates select="first_name" mode="person"/>
</xsl:copy>
</xsl:template>
<xsl:template match="first_name" mode="person">
<xsl:variable name="surname" select="following-sibling::last_name[1]"/>
<xsl:if test="normalize-space() and normalize-space($surname)">
<person>
<xsl:apply-templates select=". | $surname"/>
</person>
</xsl:if>
</xsl:template>
<xsl:template match="first_name">
<given_name>
<xsl:apply-templates/>
</given_name>
</xsl:template>
<xsl:template match="last_name">
<surname>
<xsl:apply-templates/>
</surname>
</xsl:template>
</xsl:stylesheet>
https://xsltfiddle.liberty-development.net/94AbWAW/1