Specify merging of resource-jsonld

[progval]

We will soon run into a case where the core will loose data because it does not merge resource-jsonld graphs.

Things to consider:

• Data for a Thing might not be at the root level of a resource
• What to do if data are in different responses?
• and in answers in different queries? (ie. question answering won't be stateless anymore)

[Tpt]

Merge of JSON-LD graphs is, I think, a problem large enough to be the topic of more than one research paper. I'll try to see if there is a bibliography on the subject and think about a solution. But it may take a while.

[progval]

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