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Section 2.2.1.1 Type Definition Hierarchy reads: "[Definition:] Except for ·xs:anyType·, every ·type definition· is, by construction, either a ·restriction· or an ·extension· of some other type definition. The graph of these relationships forms a tree known as the Type Definition Hierarchy." which implies that xs:anyType doesn't have a base type, since it makes xs:anyType a exception, and states that the type graph is a tree, and as far as I know circular trees does not exist(at least no one grows over here in Scandinavia). However, section 3.4.7 Built-in Complex Type Definition specifies the {base type definition} for xs:anyType to be "itself", which implies circularity. To me it seems that section 3.4.7 is wrong and its {base type definition} should be "none".
On the telcon, the WG decided to change prose in and around the definition from 2.2.1.1, but to decline to make any deeper change.
Telcon 2008-11-21 Not really a bug, just a subtlety of the wording. While we can no longer recall the reason for making anyType have itself as its own base, now is not a good time to be changing it. RESOLVED: Classify 6204 as Editorial, to clarify the prose in the definition of the type definition hierarchy in 2.2.1.1.
During its 2009-03-13 telecon, the schema WG adopted a proposal to address this issue. The change is to replace the "Type Definition Hierarchy" definition: "[Definition:] Except for ·xs:anyType·, every ·type definition· is, by construction, either a ·restriction· or an ·extension· of some other type definition. The graph of these relationships forms a tree known as the Type Definition Hierarchy." with the following: "[Definition:] Except for ·xs:anyType·, every ·type definition· is, by construction, either a ·restriction· or an ·extension· of some other type definition. The exception ·xs:anyType· is a ·restriction· of itself. With the exception of the loop on ·xs:anyType·, the graph of these relationships forms a tree known as the Type Definition Hierarchy with ·xs:anyType· as its root." With this change, the WG believes that the issue raised in this bug report is fully addressed. I'm marking this RESOLVED accordingly. Frans, 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.