constraining N3 prefixes

<tim_laptop> The prefixes should all come at the beginning.
<tim_laptop> I got more strict about that in my old age.

Hmm... I dunno about that. But if that's the way it's
to be, let's have a negative parser test for it, please.

-- 
Dan Connolly, W3C http://www.w3.org/People/Connolly/
D3C2 887B 0F92 6005 C541  0875 0F91 96DE 6E52 C29E