blank node scope - ISSUE-107 - resolve as in Semantics - hopefully on 20 March

ISSUE-107 concerns what to do with blank nodes.  This includes cross-graph
blank node scopes.

The current draft of Semantics
https://dvcs.w3.org/hg/rdf/raw-file/default/rdf-mt/index.html includes a
solution to blank node scoping.  I propose that this solution be adopted by
the WG as the result of the issue.

The basic idea is to introduce the notion of a blank node scope.  RDF
graphs within a single scope can share blank nodes, graphs not in the same
scope cannot!  This makes blank-node-renaming unnecessary during graph
merging.  (Of course, in a surface syntax, different blank nodes may have
the same b-node name, so these names may have to be changed when merging in
a particular syntax.)

For graphs not in the same scope, nothing changes.   For graphs in the same
scope not sharing blank nodes, nothing changes.
For graphs in the same scope sharing blank nodes, these blank nodes are
interpreted uniformly.

This last breaks a feature of RDF, that a set of graphs entails their
merge.  There is a new definition in Semantics (complete graphs) that shows
when this feature is retained.



This solution needs changes in Concepts, minimally introducing the notion
of a blank node scope, but maybe also talking about how blank node scope
can be determined by different surface syntaxes.

I suppose that there is also the issue of whether all the RDF graphs in a
dataset are always in the same blank node scope.  It may be that it is not
reasonable to say that this is the case, because datasets are already
sometimes used as if they do not share blank nodes.



peter