XSLT - Group child nodes under same parent - xslt-1.0

Question

I have the following input XML
Input XML
<bills>
<bill>
<billNo>1</billNo>
<exportType>1</exportType>
<invoice>
<serialNo>1</serialNo>
</invoice>
</bill>
<bill>
<billNo>1</billNo>
<exportType>1</exportType>
<invoice>
<serialNo>2</serialNo>
</invoice>
</bill>
<bill>
<billNo>2</billNo>
<exportType>1</exportType>
<invoice>
<serialNo>1</serialNo>
</invoice>
</bill>
<bill>
<billNo>2</billNo>
<exportType>1</exportType>
<invoice>
<serialNo>2</serialNo>
</invoice>
</bill>
<bill>
<billNo>2</billNo>
<exportType>1</exportType>
<invoice>
<serialNo>3</serialNo>
</invoice>
</bill>
</bills>
that needs to be converted to an output XML
Output XML
<bills>
<bill>
<billNo>1</billNo>
<exportType>1</exportType>
<invoice>
<serialNo>1</serialNo>
</invoice>
<invoice>
<serialNo>2</serialNo>
</invoice>
</bill>
<bill>
<billNo>2</billNo>
<exportType>1</exportType>
<invoice>
<serialNo>1</serialNo>
</invoice>
<invoice>
<serialNo>2</serialNo>
</invoice>
<invoice>
<serialNo>3</serialNo>
</invoice>
</bill>
</bills>
The following XSLT is being used for transformation however I am missing something as the child nodes under bill also get repeatedly copied instead of only the invoicechild node and its children getting copied. XSL is as below
XSL
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output omit-xml-declaration="yes" indent="yes" />
<xsl:strip-space elements="*" />
<xsl:key name="key" match="bill" use="billNo" />
<xsl:template match="node()|#*">
<xsl:copy>
<xsl:apply-templates select="node()|#*" />
</xsl:copy>
</xsl:template>
<xsl:template match="bill[generate-id() = generate-id(key('key', billNo)[1])]">
<bill>
<xsl:apply-templates select="key('key', billNo)/*" />
</bill>
</xsl:template>
<xsl:template match="bill" />
</xsl:stylesheet>

Answer 1

One way to get the expected result is to change this:
<xsl:apply-templates select="key('key', billNo)/*" />
to:
<xsl:apply-templates select="billNo | exportType" />
<xsl:apply-templates select="key('key', billNo)/invoice" />
Note that this assumes that the exportType is the same for all items grouped under a common billNo.