Revision of the sugar proposal

Named occurrences of triples as RDF reification

This syntax is simply a shorthand for RDF reification (or some variation thereof). In other words, any occurrence in the graph of a triple

<< :e | :s :p :o >> :p1 :o1 .

would be syntactic sugar for the following triples:

:e rdf-star:is-reification-of _:b
_:b rdf:subject :s .
_:b rdf:predicate :p .
_:b rdf:object :o .
:e :p1 :o1 .

with _:b a fresh new blank node, unique within the RDF graph. The abstract syntax would not need to be extended.

It is necessary to add a notion of syntactically "well-formed" RDF:

Definition: An RDF graph is reification well-formed iff:

1. The object of each rdf-star:is-reification-of triple is a blank node.
2. Any blank node appearing as object of a rdf-star:is-reification-of triple occurs only in that triple and exactly once as subject of a rdf:subject triple, a rdf:predicate triple, and a rdf:object triple.

The notion of well-formedness does not impact reification-star-free RDF 1.1 graphs, and so it is fully backwards compatible.