5.1 Constructor Functions for XML Schema Built-in Types
'The semantics of the constructor function " xs:TYPE(arg) " are identical to the semantics of " arg cast as xs:TYPE? ", except for constructors for xs:QName and types derived from xs:NOTATION which are identical to "cast as xs:TYPE ".'

--- Presumably, that last snippet is missing "arg". That is, it should be
         arg cast as xs:TYPE

--- Note that the XQuery doc (3.12.5) doesn't include the exception for
    xs:QName and types derived from xs:NOTATION.

--- But it seems to me that the exception isn't necessary. Two paragraphs later,
    we're told that, in the cases covered by this exception, the argument is
    constrained to be a string literal. Thus, the cast expression's atomization
    is guaranteed to result in a single string value, in which case the result
    of
        arg cast as xs:TYPE
    is the same as
        arg cast as xs:TYPE?
    Isn't it?