[Versa] EBNF round 3

[Chimezie Ogbuji]

This is a tough one (or maybe not, given our pattern of using qnames
for functions and variables).  Should context's be qnames or strings? 
There is no standard school of thought on this or precedent.  In the
EBNF, they are Qname which suggests they are URIs.  I think it's safe
to assume that contexts should be URIs since you might want to make
statements about them (in the way we do when we make statements about
repository RDF document ftss URIs).

Chimezie