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In defining the dynamic context for XPath evaluation (section 3.13.4.2) we say that available documents is empty, but available collections and the default collection are implementation defined. There is no logic to making these different. Either all three should be empty, or all three should be implementation defined. I think there are many use cases for writing assertions that make access to external documents; for example one can check that the value of an EMPNR attribute corresponds to a value that is present in some external document or database. However, there are also many difficulties - in particular it means that the validity of the attribute depends heavily on the environment in which validation is performed. Making the values of these context properties implementation-defined would pass the buck to implementors to solve these problems if they can find a way of doing it; I would be inclined to favour this route over banning it out of hand.
RESOLUTION: change available collections and default collection to empty. Status editorial, needs drafting.
During its 2009-03-13 telecon, the schema WG adopted a proposal to address this issue. Changes include: 1. Replaced "implementation-defined" with "the empty set" for item 9 (available collections) in 3.13.4.2 XPath Evaluation. 2. Replaced "implementation-defined" with "the empty sequence" for item 10 (default collections) in 3.13.4.2 XPath Evaluation. With these changes, the WG believes that the issue raised in this bug report is fully addressed. I'm marking this RESOLVED accordingly. Michael, as the persons who opened and reopened this issue, if you would indicate your concurrence with or dissent from the WG's disposition of the comment by closing or reopening the issue, we'll be grateful. If we don't hear from you in the next two weeks, we'll assume that silence implies consent.