ESI Language Specification 1.0 feels incomplete

Hi!
Not sure if this is the right place to post this, so sorry if it's not.
I am currently interested in implementing the ESI ("Edge Side Includes")
specifications, and so I've been reading http://www.w3.org/TR/esi-lang
carefully.

But this document feels incomplete, as it tries to define a language
without one line of EBNF grammar.
I can understand it for the part that is just XML, but for section 5
("ESI Expressions") it feels like some precise grammar should be
included, as right now it is very vague.

I would have loved to be able to submit some EBNF, but the truth is I
have very little experience with it. I made an attempt
(https://gist.github.com/hectorj/bbbfd6a3be6e2d72ed04) but I can bet
it's wrong.
I also tried to see if someone on the web had already defined that
grammar but did not find anything.

So I'm asking for help : does someone already has an EBNF description of
ESI Expressions or is experienced enough with EBNF to write it ?