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It seems that the description of the fn:path function should say what errors are raised in the case that $arg is not provided and the context item is absent or not a node. (Similar to, for example, the fn:root function) A section could be inserted: Error Conditions The following errors may be raised when $arg is omitted: If the context item is absent, dynamic error [err:XPDY0002] If the context item is not a node, type error [err:XPTY0004].
The Working Group agrees that these error conditions should be specified. These are, of course, the same errors that XPath would use for the same conditions in general.
The change has been applied.