I mispoke here a bit about property sets. HyTime doesn't extend the SGML
property set, it defines additional property sets that define objects and
properties understood by a HyTime processor. Any application or
architecture can do the same thing. The SGML Extended Facilities (Annex A
to the corrected HyTime standard), does add a module to the SGML property
set in order to support those facilities, but it does not modify the
existing property set.
Cheers,
E.