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The rules for the subtype() relation say The judgement subtype(A, B) determines if the sequence type A is a subtype of the sequence type B. A can either be empty-sequence(), xs:error, or an ItemType, Ai, possibly followed by an occurrence indicator. (and then say the same for B) It's not clear whether the second sentence should parse as A is (a) one of (empty-sequence(), xs:error, or an ItemType, Ai), followed by (b) an optional occurrence indicator. or as A is one of (a) empty-sequence(), (b) xs:error, or (c) an ItemType Ai followed by an optional occurrence indicator. Actually I think it's neither of these. I think xs:error is itself an ItemType, so it doesn't need to be listed; and it can be followed by an occurrence indicator, so the actual rule is A is either (i) empty-sequence(), or (ii) an ItemType Ai followed by an optional occurrence indicator.