This comment on line 217 is not correct:
   <!-- pre uses %Inline excluding big, small, sup or sup -->

These rules state otherwise:
   <!ENTITY % pre.content
      "(#PCDATA | a | %fontstyle; | %phrase; | %special.pre; | %misc.inline;
       | %inline.forms;)*">
   <!ENTITY % phrase "em | strong | dfn | code | q |
                      samp | kbd | var | cite | abbr | acronym | sub | 
sup ">

IOW sub and sup elements are allowed according to the entities, so if 
the intention was otherwise, it should be corrected.
-- 
liorean <mailto:liorean@user.bip.net>

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