[UltraSane]

allowing edges to have edges is something that RDF* allows.

Property graphs DBs like Neo4j don't support it but you can do it by using a node as a relationship. This is called a metanode or a hypernode. The need for this is mitigated somewhat by the fact that property graphs allow edges to have properties themselves. so you would use

(Alice)-[:LIVES_IN{valid_from:2020,valid_to:2024}]-(USA)

Edit: Read the paper and it is actually an attempt to unify the RDF, RDF* and property graph data models which is VERY interesting.