Unable to group the element value using group by in XSLT - xslt-2.0

Question

I have an XML lie below:
<Products>
<Product1>
<Reference>000510143244</Reference>
<Value1>543</Value1>
</Product1>
</Products>
<Products>
<Product1>
<Reference>000510143244</Reference>
<Value1>543</Value1>
</Product1>
</Products>
<Products>
<Product1>
<Reference>45768799322</Reference>
<Value1>543</Value1>
</Product1>
</Products>
<Products>
<Product2>
<Reference>35726318090</Reference>
<Value1>543</Value1>
</Product2>
</Products>
<Products>
<Product2>
<Reference>35726318090</Reference>
<Value1>543</Value1>
</Product2>
</Products>
I want to get only first value of the Product1 reference...but I am unable to get that.Also it is not mandatory that Product 1 will always be the first element in input xml.
Any suggestions how can I get that?
I have tried to get the value as :
<xsl:template match="//Products">
<xsl:variable name="Product1">
<xsl:for-each-group select="/Reference" group-by="/Reference">
<xsl:copy-of select="." />
</xsl:for-each-group>
</xsl:variable>
</xsl:template>
Update:1
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="2.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="xml" encoding="UTF-8" indent="yes" />
<xsl:template match="Products[child::Product1][1]">
<xsl:value-of select="." />
</xsl:template>
<xsl:template match="text()" />
</xsl:stylesheet>
My expected output is :000510143244

Answer 1

To get the first occurrence of <Products> who has <Product1>, you might need to match the parent tag or root tag of your input XML.
Assuming your input as below:
<?xml version="1.0" encoding="UTF-8" standalone="yes" ?>
<root>
<Products>
<Product2>
<Reference>35726318090</Reference>
</Product2>
</Products>
<Products>
<Product1>
<Reference>02563899183</Reference>
</Product1>
</Products>
<Products>
<Product1>
<Reference>000510143244</Reference>
</Product1>
</Products>
<Products>
<Product1>
<Reference>000510143244</Reference>
</Product1>
</Products>
<Products>
<Product2>
<Reference>35726318090</Reference>
</Product2>
</Products>
</root>
The following code can give you the result:
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="2.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="xml" encoding="UTF-8" indent="yes" />
<xsl:template match="root">
<xsl:for-each-group select="Products/Product1" group-by="Reference">
<xsl:copy-of select="current-group()[1]" />
</xsl:for-each-group>
</xsl:template>
</xsl:stylesheet>
See the demo: https://xsltfiddle.liberty-development.net/3NJ38Zx
Update:
OR you can simply achieve it by following code:
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="2.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="xml" encoding="UTF-8" indent="yes" />
<xsl:template match="Products[child::Product1][1]">
<xsl:copy-of select="." />
</xsl:template>
<xsl:template match="text()" />
</xsl:stylesheet>
Update 2:
<xsl:template match="root">
<xsl:variable name="ref">
<xsl:for-each-group select="Products/Product1" group-by="Reference">
<xsl:copy-of select="current-group()[1]/Reference" />
</xsl:for-each-group>
</xsl:variable>
<xsl:value-of select="$ref"/>
</xsl:template>
https://xsltfiddle.liberty-development.net/3NJ38Zx/1
Update 3:
You cannot assign a value to global variable from a template.
There are two ways to get what you required.
1) Create a global variable as below which will take first <Products> whose child element is <Product1> and will display it's Reference
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="2.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="xml" encoding="UTF-8" indent="yes" />
<xsl:variable name="ref" select="root/Products[child::Product1][1]/Product1/Reference" />
<xsl:template match="/">
<xsl:value-of select="$ref" />
</xsl:template>
</xsl:stylesheet>
2) You can modify the template as below to get the result.
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="2.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="xml" encoding="UTF-8" indent="yes" />
<xsl:template match="Products[child::Product1][1]/Product1/Reference">
<xsl:value-of select="." />
</xsl:template>
<xsl:template match="text()" />
</xsl:stylesheet>