root

The root() function returns the root node of the tree containing a specified start node, or the root of the tree containing the context node.

Argument	Data Type	Meaning
start-node (optional)	node()?	A node in the tree whose root is required. If the argument is omitted, it defaults to the context node. It is then an error if the context item is not a node (for example, if it is an atomic value, or if it is undefined)
Result	node()?	The root of the tree containing the start node