Returns a string that uniquely identifies the node in the argument node-set that is first in document order.
string generate-id(node-set?)
The unique identifier must consist of ASCII alphanumeric characters and must start with an alphabetic character. Thus the string is syntactically an XML name. 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.