Two documents are treated as the same document if they are identified by the same URI. The URI used for the comparison is the absolute URI into which any relative URI was resolved and does not include any fragment identifier. One root node is treated as the same node as another root node if the two nodes are from the same document. Thus, the following expression will always be true:

generate-id(document("foo.xml"))=generate-id(document("foo.xml"))

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This was confusing me why this would be in XSLT not in XPath (i.e fn:doc() is declared in XPath so both  XSLT and XQuery can use them.

<poke>

Found it !

http://www.w3.org/TR/xpath-functions/#func-doc

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By default, this function is http://www.w3.org/TR/xpath-functions/#stable. Two calls on this function return the same document node if the same URI Reference (after resolution to an absolute URI Reference) is supplied to both calls. Thus, the following expression (if it does not raise an error) will always be true:

doc("foo.xml") is doc("foo.xml")