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The specification for fn:data() with zero arguments (new in 3.0) states that error XPTY0004 is raised if the context item is not a node. This contradicts the statement that "the behavior of the function if the argument is omitted is exactly the same as if the context item had been passed as the argument", since data(.) works perfectly well when the context item is an atomic value.
I have marked this as editorial and have fixed it by removing the error condition.