grammar extraction

During last f2f, we discussed about the possibility to isolate common part 
of the grammar between header syntax and URI syntax.
However the common part would be external definition and at most the time 
part without the units, like 'frametime' or 'clocktime' (the unit in the 
header needs a specific entry to avoid implied LWS).

So it is worth doing? (Jack, any input?)

(tracker this is about ACTION-174)
-- 
Baroula que barouleras, au tiéu toujou t'entourneras.

         ~~Yves