proposal for ISSUE-107 (was Re: [All] Agenda for 3 April 2013)

I have copied the relevant parts of Concepts and Semantics for blank node
scope below.   My very minimialist suggestion is to resolve ISSUE-107:
Revised definition of blank nodes by making the removals / replacements
noted via [.../...] below.

This proposal is very minimalist.  It does cover the formal aspects of blank
nodes and blank node identifiers, mostly by saying nothing.

It would probably be a good idea to include a paragraph in Concepts 3.4
saying that in most circumstances different RDF graphs do not share blank
nodes, but that some RDF syntaxes and RDF tools do allow different RDF
graphs to share blank nodes; and a note that this is a change from the 2004
situation to align with practice.  Either Concepts or Primer could provide
examples.

Concepts

3.4 Blank Nodes

Note: Blank node identifiers are local identifiers that are used in some
concrete
RDF syntaxes or RDF store implementations. They are [always locally] scoped
to
the [file/document] or RDF store, and are not persistent or portable
identifiers for
blank nodes. Blank node identifiers are not part of the RDF abstract syntax,
but are entirely dependent on the concrete syntax or implementation. The
syntactic restrictions on blank node identifiers, if any, therefore also
depend on the concrete RDF syntax or implementation.

3.5 Replacing Blank Nodes with IRIs

Blank nodes do not have identifiers in the RDF abstract syntax. The blank
node identifiers introduced by some concrete syntaxes have only local scope
and are purely an artifact of the serialization.



Semantics

3 Notation and Terminology

Any set of graphs can be treated as a single graph simply by taking the
union of the sets of triples. If two or more of the graphs share a blank
node it will retain its identity when the union graph is formed. [Graphs can
share blank nodes only if they are derived from graphs described by
documents or surface structures which share a single scope for blank node
identifiers.]

4 Simple Interpretations

Mappings from blank nodes to referents are not part of the definition of an
interpretation, since the truth condition refers only to some such
mapping. Blank nodes themselves differ from other nodes in not being
assigned a denotation by an interpretation, reflecting the intuition that
they have no 'global' meaning [outside the scope in which they occur].

Intuitive summary

An RDF graph is true exactly when:
1. the IRIs and literals in subject or object position in the graph all
refer to things,
2. there is some way to interpret all the blank nodes in the [scope/graph]
as
referring to things,
3. the IRIs in property position identify binary relationships,
4. and, under these interpretations, each triple S P O in the graph asserts
that the thing referred to as S, and the thing referred to as O, do in fact
stand in the relationship identified by P.




peter