Returns the root of the tree to which $arg belongs. This will usually, but not necessarily, be a document node.
If $arg is the empty sequence, the empty sequence is returned.
If $arg is a document node, $arg is returned.
If the function is called without an argument, the context item is used as the default argument. If the context item is undefined an error is raised: [err:XPDY0002]. If the context item is not a node an error is raised: [err:XPTY0004].
var doc = xdmp.unquote('<a><quantity>5.0</quantity></a>')
.toArray()[0];
fn.root(doc);
=>
<?xml version="1.0" encoding="UTF-8"?>
<a><quantity>5.0</quantity></a>