Existential blank nodes in LDTab

[ckindermann]

One of LDTab's guiding design choices is to eliminate blank nodes where possible. In the case of existential blank nodes, i.e., blank nodes that do not appear as objects in an RDF graph, we can choose to

1. collapse the blank node in the usual way:
   i) introduce a 'wiring' blank node for the subject column
   ii) use "unknown" for the predicate (or a value of a property that indicates what kind of structure the blank node points to - so far we have chosen its rdf:type, if available.)
   iii) put the JSON structure of the existential blank node in the object column.

OR

2. unfold the first level of the JSON structure created in 1. iii) into LDTab rows.

Example:

Consider a disjoint classes axiom in OWL. This is represented with an existential blank node in RDF corresponding to the following JSON object in LDTab:

{
    "owl:members": {
        "object": [
            {
                "datatype": "_IRI",
                "object": "<http://purl.obolibrary.org/obo/BFO_0000009>"
            },
            {
                "datatype": "_IRI",
                "object": "<http://purl.obolibrary.org/obo/BFO_0000018>"
            },
            {
                "datatype": "_IRI",
                "object": "<http://purl.obolibrary.org/obo/BFO_0000026>"
            },
            {
                "datatype": "_IRI",
                "object": "<http://purl.obolibrary.org/obo/BFO_0000028>"
            }
        ],
        "datatype": "_JSONLIST"
    },
    "rdf:type": {
        "object": "owl:AllDisjointClasses",
        "datatype": "_IRI"
    }
}

Instead of putting this JSON object in LDTab's object column (as per 1. above), we would unfold the top level of the JSON object into the following two LDTab rows (I'm using CURIEs instead of full IRIs for classes here):

subject	predicate	object	datatype
wiring:blanknode:_	owl:members	[{"datatype":"_IRI","object":"obo:BFO_0000009"},{"datatype":"_IRI","object":"obo:BFO_0000018"},{"datatype":"_IRI","object":"obo:BFO_0000026"},{"datatype":"_IRI","object":"obo:BFO_0000028"}]	_JSONLIST
wiring:blanknode:_	rdf:type	owl:AllDisjointClasses	_IRI