I don't have any comment on the issue of language tags, but speaking as a reasonably avid ABNF hacker, I agree with Sam, and would not want to establish a convention that ABNF in IETF RFCs is expected to be precise.
The counter-argument is the all-too-frequent occurance when you deal with willful cretins who will *insist* that the specification says such-and-such when it really says the opposite, and will leap upon the most bizarre interpretation of text in order to bolster their arguments.
This is unavoidable; however, it helps a lot if the ABNF firmly comes down on the side of the good guys.
I've spent entirely too much of my life in the past few years fending off cretins, to agree knowingly to anything that makes me more vulnerable to them in the future.
Nor do "gentlemen's agreements" work any more. We may all be (ladies and) gentlemen here, but "out there" there are individuals who are not.
As painful as the process may be, I believe that the ABNF should be as tight as possible, preferably by ABNF rules but at least through ABNF comments.
However, be careful about comments. I had one cretin insist that "between n and m inclusive" (where n and m were integers) had an implied restriction that n <= m, and that when n > m it meant an empty set of values.
-- Mark --
http://staff.washington.edu/mrc Science does not emerge from voting, party politics, or public debate. Si vis pacem, para bellum.
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