YAML diagram: allow "objects as keys"

[VladimirAlexiev]

Unlike JSON, YAML keys are not limited to strings, but can be objects.

json-ld/yaml-ld#45 discusses using this for RDF-star, which allows putting annotations on statements.
Here are 2 examples. I use the JSON-like "flow" notation (YAML is really an extension of JSON), if needed I'll restate it in "block" notation:

• {$id: bob, age: 42}: {certainty: 0.8, source: Newspaper-1}: "Bob's age is 42, with a certainty of 0.8, as stated in a Newspaper"
• {{$id: bob, knows: {$id: alice}}: {certainty: 0.8}}: {claims: ted}: Ted claims that the certainty of Bob knowing Alice is 0.8"

I have zero drawing skills but I'll describe how I'd like the second example to be drawn:

• draw {$id: bob, knows: {$id: alice}} as usual and put it in a box
• draw a line from the first box to a second box with {certainty: 0.8}
• put both in a bigger box
• draw a line from the big box to a fourth box with {claims: ted}

[mfhepp]

This could be useful for many typical needs in the context of ontology documentation, Linked Open Data, Knowledge Graphs, etc.!

However, note that in the original YAML-LD issue, there are objections against this proposal, too, claiming that there is already a proper way to express this in YAML-LD, see @gkellogg in json-ld/yaml-ld#45 (comment).

So before moving this forward, it would be good to know if there are use cases for this in plain YAML beyond this extension proposal for YAML-LD.