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.