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+++ This bug was initially created as a clone of Bug #5074, to allow an issue reported for 1.1 to be tracked for 1.0 also +++ The substitution group definition (below) says member names can vary widely but that member types are "strictly limited". This may be technically true, but seems to suggest a false sense of security in use of substitution groups. By default, with no derivation constraints, a substitution group can transitive include wildly different types. That is, a restriction step can remove all optional content and an extension step could introduce completely different content. If my analysis is correct, please consider a milder statement and/or a warning about unintentional extensibility. --------------------- 2.2.2.2 Element Substitution Group All such members must have type definitions which are either the same as the head's type definition or restrictions or extensions of it. Therefore, although the names of elements can vary widely as new namespaces and members of the ·substitution group· are defined, the content of member elements is strictly limited according to the type definition of the ·substitution group· head.
Adding Xan Gregg to CC list, as the originator of the comment (w.r.t. 1.1).