<!ENTITY Chris PUBLIC "-//EBT//NONSGML Christopher R. Maden//EN" SYSTEM>
The only reason I could see for not allowing the entity overrides is
that it requires DTD parsing
This is true for any entities at all except the default ones.
So this would be an argument for not allowing entities other than
those in a defult set.
Also, note that entities can be declared in the subset even if nothing
else is -- e.g.
<!DOCTYPE IRISH-HISTORY SYSTEM "irish history.dtd" [
<!Entity papist "Roman Catholic">
<!Entity prod "Protestant">
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