fn:root

root() ➔ node()

Returns the root node (typically but not necessarily a document node) of the tree containing the context node.

There are no arguments
Result		node()

root($arg as node()?) ➔ node()?

Returns the root node (typically but not necessarily a document node) of the tree containing the supplied node.

Arguments
	$arg	node()?	A node in the tree whose root is required
Result		node()?

http://www.w3.org/2005/xpath-functions