How to group XML data by multiple value

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I need to group "Document" value from XML. Problem is because key value (productType) can be multiple.This is XML:\[code\]<Documents><Document> <id>1</id> <title>title1</title> <productTypes> <productType id="x1">Capital Costs Analysis Forum - Brazil</productType> <productType id="x3">Environmental, Health and Safety & Sustainability</productType> </productTypes></Document><Document> <id>2</id> <title>title2</title> <productTypes> <productType id="x1">Capital Costs Analysis Forum - Brazil</productType> </productTypes></Document><Document> <id>3</id> <title>title3</title> <productTypes> <productType id="x3">Environmental, Health and Safety & Sustainability</productType> </productTypes></Document><Document> <id>4</id> <title>title4</title> <productTypes> <productType id="x2">Defense, Risk & Security</productType> </productTypes></Document>\[/code\]And this is what I try:\[code\]var documents = from document in some.Descendants("Document") group document by (string)document .Element("productTypes") .Elements("productType") .First() into docGroupselect docGroup;\[/code\]My code is working only if is there one productType element. How to change my code to work if there is multiple value of productType?