[Bug 19425] [XP30] Substitution group membership is non-transitive

https://www.w3.org/Bugs/Public/show_bug.cgi?id=19425

--- Comment #3 from Michael Kay <mike@saxonica.com> ---
Saxon requires the membership of a substitution group to be known when a module
containing a reference to the head of the SG is compiled (i.e. it prevents
anyone adding to the substitution group from that point on). Other
implementations might allow more dynamic flexibility, but then they cannot make
assumptions based on the SG membership being statically known. The spec leaves
implementations to decide how flexible to be.

I don't think that this is really any different from being able to decide
whether or not an element E may or may not be passed to a function expecting
schema-element(H). If you freeze the SG membership at compile time, you can
type-check this statically; if you don't, then you can't. The spec allows both
options.

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