XSLT group on child node - group-by

Question

I'am try to convert XML on group of child nodes. Main information is in bill node, it must be group by bill numbers.
original XML, this is simplified version of original XML
<items>
<item>
<bill>10</bill>
<name>first (10)</name>
<price>111</price>
</item>
<item>
<bill>10</bill>
<name>second (10)</name>
<price>222</price>
</item>
<item>
<bill>10</bill>
<name>third (10)</name>
<price>333</price>
</item>
<item>
<bill>11</bill>
<name>first (11)</name>
<price>1</price>
</item>
<item>
<bill>11</bill>
<name>second (11)</name>
<price>2</price>
</item>
</items>
final file
<bills>
<bill>
<number>10</number>
<items>
<item>
<nameitem>first (10)</nameitem>
<priceitem>111</priceitem>
</item>
<item>
<nameitem>second (10)</nameitem>
<priceitem>222</priceitem>
</item>
<item>
<nameitem>third (10)</nameitem>
<priceitem>333</priceitem>
</item>
</items>
</bill>
<bill>
<number>11</number>
<items>
<item>
<nameitem>first (11)</nameitem>
<priceitem>1</priceitem>
</item>
<item>
<nameitem>second (11)</nameitem>
<priceitem>2</priceitem>
</item>
</items>
</bill>
</bills>
there is working XSLT, for basic grouping, but I have not idea, how to build another structure inside bill node, based on final XML
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="xml" version="1.0" encoding="UTF-8" indent="yes"/>
<xsl:strip-space elements="*"/>
<xsl:key name="group-by-bill" match="item" use="bill"/>
<xsl:template match="items">
<bills>
<xsl:for-each select="item[generate-id()=generate-id (key('group-by-bill', bill)[1])]">
<bill number="{bill}">
<xsl:copy-of select="key('group-by-bill', bill)"/>
</bill>
</xsl:for-each>
</bills>
</xsl:template>

Answer 1

Use this:
<xsl:template match="items">
<bills>
<xsl:for-each select="item[generate-id()=generate-id (key('group-by-bill', bill)[1])]">
<bill>
<number><xsl:value-of select="bill"/></number>
<items>
<xsl:for-each select="//item[bill = current()/bill]">
<item>
<xsl:copy-of select="name|price"/>
</item>
</xsl:for-each>
</items>
</bill>
</xsl:for-each>
</bills>
</xsl:template>
See Transform at https://xsltfiddle.liberty-development.net/jyH9rM5