The generate-id() function generates a unique ID for a given node.
string generate-id(node-set[optional])
Optionally, a node-set. If no node-set is specified, the context node is used. An empty node-set produces an empty string. A node-set with multiple nodes produces an ID for the node that is first in document order.
If the node-set is not empty, a unique ID in the form of an XML name. This ID is guaranteed to be the same no matter how many times it is generated in the course of a single transform, but not beyond that. Different nodes are guaranteed to have different generated IDs. The processor does not check to see if there are any other XML names that happen to be in the document that inadvertently match the generated ID, an unlikely occurrence, but theoretically possible.
Defined: XSLT, section 12.4.
