I sympathise with this :-).  Parameter Entities (PEs) initially gave parser
writers and the ERB/WG a lot of problems.  The current rules (I refer to 970331)
are simpler than initially, but I must admit that I don't find that particular
part of the spec easy to understand.  I think it's fair to say that
**apart from PEs** the DTD is easy to parse.  It may also be that the current
rules for PE substitution can be described in a simple way and I just haven't
picked this up.
I am sure that you will get answers from people more knowledgeable than me,
but also remember that a revision of the spec is due on July 1.  What is in
it is determined by the ERB, but I would be surprised if there were not 
clarifications relating to PEs :-).
	P.
-- Peter Murray-Rust, domestic net connection Virtual School of Molecular Sciences http://www.vsms.nottingham.ac.uk/