Introduce hasTopConcept from skos

[azaroth42]

The National Gallery has a need to get from a locally managed concept to the top of its hierarchy, as opposed to the immediate parent (which is handled by broader).

There's already a way to model the ConceptScheme (a Set in linked art) which then the Types are member_of. This doesn't directly help as you can't get from the Set to the "top" concept, and some schemes have multiple top concepts, including our commonly used AAT. So there's no way, without traversing an arbitrary number of hops to find the top concept.

Proposal to introduce skos:hasTopConcept as topConcept, with a domain and range of E55_Type.

The only counter argument I can come to is that it /is/ computable, and thus could be a HAL link to do the search ... but the result of HAL links is a result set, not an instance as this is a special case where there can only be one top concept per concept.

[aisaac]

+1
And in fact about the assumed computability, there's this: https://www.w3.org/TR/skos-reference/#L2446
I'm not 100% sure anymore now, but I suppose we left things a bit open, because there were some corner cases of "sub-schemes" that could have a concept as entry point, which would have a broader concept in the encompassing super-scheme. This was at the time the SKOS group was instructed that there were cases of "micro-thesauri" and beasts like this, which may have been represented as ConceptScheme.