Re: Adaptation of the semantics

I forgot to put the link to the preview, for those not quite familiar 
with our github:

https://pr-preview.s3.amazonaws.com/w3c/rdf-star/pull/127.html#rdf-star-semantics

On 05/03/2021 19:00, Pierre-Antoine Champin wrote:
>
> Hi all,
>
> I just pushed a pull-request adapting the semantics:
>
> https://github.com/w3c/rdf-star/pull/127
>
> I believe it has some advantages over the current version:
>
>   * it does not rely anymore on "hidden" predicates (see issue #101
>     <https://github.com/w3c/rdf-star/issues/101>)
>   * it does not have the "merging" issue warned about in §6.3.1
>     <https://w3c.github.io/rdf-star/cg-spec/2021-02-18.html#combining-rdf-star-graphs>
>   * I think that it allows us to align SPARQL query semantics with
>     simple entailment (as newly defined)
>   * I think that it allows the Interpolation Lemma
>     <https://www.w3.org/TR/rdf11-mt/#dfn-interpolation> to extend to
>     RDF-star
>
> (I didn't formally prove the last two items, hence "I think"...)
>
> The trick is that we do not map anymore RDF-star graphs to a single, 
> semantically equivalent RDF graph.
> Instead, we map it to a pair of RDF graphs, which can be thought of as 
> a "lower and upper bound" of the RDF-star graph, in terms of 
> entailment. The semantics of the RDF-star graph is defined through the 
> semantics of its "bounds", reusing RDF semantics as is (as we 
> currently do).
>
> In this new semantics, a strict RDF-star graph (i.e. one that contains 
> embedded triples) has no exactly equivalent RDF graph, so it still can 
> not be conveyed exactly using RDF syntaxes (but we do not rely anymore 
> on hidden predicates for that). However, either of the two "bounds" 
> can be used to approximate the RDF-star graph in legacy RDF. The 
> "lower bound" will produce correct but incomplete inferences. The 
> "upper bound" will produce complete inferences, with a few spurious 
> (but generally harmless) ones.
>
> I am curious to get some feedback on this.
>
>