
The function returns the argument string with whitespace
normalized by stripping leading and trailing whitespace and
replacing sequences of whitespace characters by a single space.
Whitespace characters are the same as those allowed by the S production in XML,
i.e. space (#x20), carriage returns (#xD), line feeds (#xA), and tabs (#x9).
If the argument is omitted, it defaults to the context node converted to a string,
in other words the string-value of the context node.
