The purpose of this function is to generate a string of characters that uniquely identifies a node. Such values are useful for attributes of type ID.
The function takes one optional argument, a node-set:
S is a node-set. If
S is not given, the function
generates an identifier for the context node. If
S is empty, the function
returns an empty string. If
contains more than one node, the function operates on the
one that occurs first in the document.
Within a given execution, this function will always produce the same value for a given node. There is no guarantee that it will produce the same value on a different execution of the stylesheet.