[lojban] Lojban CFG Questions

[Roman Naumann <roman_naumann@fastmail.fm>]

Hello dear lojbanistanians,

i'm starting with an attempt to find a CFG for lojban - or if it shows 
impossible, to prove it being impossible (so that you see what exactly is 
impossible and thus to improve).

Back in 2008 when there was a challenge [1] to do exactly that, i would have 
done it, but i lacked the knowledge to proof or proof-wrong grammars. Learned 
it during the last year at university and thought, hey, why not put it to use.

However, i realized just now looking at the EBFN [2] causes eye cancer. Also, 
i've never been good with lojban at all. I don't get the (formalizing) problem 
with elidable terminators yet. To get started, it would be extremely helpful 
to work on an abstraction. I'd be glad if you could provide such to me. To 
give you an idea what kind of abstraction i have in mind, here's an example 
(though perhaps not very useful):

We have five kinds (sub)sentences. They start and terminate with 'a', 'b', 'c', 
'd', 'e'. Inside a (sub)sentence, only subsentences with a letter later in 
alphabet may stand. ("a c c a" is thus valid, "b a a b" is invalid [whitespace 
ignored], as a..a is no valid subsentence of b..b).
Inside of sentences may (beside any number of subsentences) stand zero or more 
of numbers (which are our abstraction of words). Each number starts with zero 
and may not contain further zeros (this is to spare us the necessity for 
whitespace). Terminators [a-e] may be elided, if directly followed by another 
terminator. Thus, a implicitly terminates [b-e] subsentences, b implicitly 
terminates [c-e] subsentences and so on.
A valid example 'word' of the language is: "a 01 02 c 0 e c 08 04 a"
It should parse to: """ a(01 02 c(0 e())c 08 04)a """
(didn't want to draw a parse tree, but it this this is enough to get the 
point)

So, do you think this abstraction catches the elidable terminator problem or 
is it too simple? If it's too simple, why, what's missing?
Besides elidable terminators, are there other problems why you think lojban 
can't be expressed as a CFG (without the grammar being way too large)?

Regards,
Roman

[1] http://www.mail-archive.com/lojban-beginners@lojban.org/msg04337.html

[2] http://www.lojban.org/publications/formal-grammars/bnf.300.txt

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