Use rules to conveniently infer a tree of triples

[philharveyonline]

We're building a system that uses SHACL-AF rules to infer triples.

We need a convenient and efficient way to infer a tree of triples.

In @HolgerKnublauch 's database example from https://github.com/w3c/data-shapes/wiki/Proposal-on-Node-Expressions-for-SHACL-Core-1.2, an edg:tableCount property is inferred. But we need to infer something analogous to this:

edg:Database1 edg:hasStatistics edg:Statisics1 .
 
edg:Statisics1           # a new IRI, probably generated from a Function using a random UUID
  edg:tableCount "2" ;   # inferred from the count of tables
  edg:userCount "42" .   # inferred from the count of users

We don't want to use SPARQL Node Expressions, for the reasons mentioned in the wiki page above and in #215.

We could achieve this using multiple runs of the SHACL Processor, firstly to generate edg:Database1 edg:hasStatistics edg:Statisics1, and secondly to generate the edg:Statisics1 xxx yyy triples. But that's too slow for our requirements.

So to make this tree of triples we need a way to use the result of a Node Expression (the one that generates egs:Statistics1 above) in multiple inferred triples, as the object of some and the subject of others.

We may invent our own new rule types and/or expression types to achieve this, but think it may be a common enough requirement to include in the Node Expression spec.

(PS I don't have permission to edit issue labels in this repo, but request that this issue to be labelled "Inferencing and "UCR" please.)

[ajnelson-nist]

I suspect there's a technical challenge here with constructing inner-tree-nodes' names. I have started discussion on a potential way forward for that part in #228 .