function selects elements by their unique ID. When the argument to
is of type node-set, then the result is the union of the result of applying id to the string-value of each of the nodes in the argument node-set. When the argument to
is of any other type, the argument is converted to a string as if by a call to the string function; the string is split into a whitespace-separated list of tokens (whitespace is any sequence of characters matching the production S); the result is a node-set containing the elements in the same document as the context node that have a unique id equal to any of the tokens in the list.