
The function returns a string that uniquely identifies the first node in
the place argument node-set.
The unique identifier will consist of ASCII alphanumeric characters and will start with an alphabetic character.
Thus, the string is syntactically an XML name.
It always generates the same identifier for the same node.
It always generates different identifiers from different nodes.
This function is under no obligation to generate the same identifiers each time a document is transformed.
There is no guarantee that a generated unique identifier will be distinct from any unique IDs specified in the source document.

If the argument node-set is empty, the empty string is returned.

If the argument is omitted, it defaults to the context node.
