> > 2008-08-08/yueu
> I have no idea what "yueu" means.
Formally, it is out of today's BNF. But it wouldn't be counter-intuitive 
to introduce it there. In analogy with #312 to #314, I was thinking of "u" 
as "unspecified" which could be combined with the BNF's definition of 
longYear as

> longYear = "y" "-"? positiveInteger "e" positiveInteger | [...]

Thus, we get "y" (for longYear) "u" (for unspecified) "e" (for exponent) 
"u" (for unspecified). The result is "yueu", that is a longYear with 
unspecified digits (including those coming after the "e"), which 
semantically has some similarities with "unknown".

Limiting the introduction of new "parts" in the EDTF BNF keeps it simple.
Such a notation consists of "known parts" and would therefore be easily 
machine-interpreted using a (very slightly) modified version of today's 
BNF. It is also logical and easy to memorize. That's what I mean with 
"explains itself".