My point is that I think I7 is trending towards multiple independent relations anyway, ie. multiple trees. Which implies that I6 is in the wrong by trying to smoosh together containers and supporters in the same tree.
Though yes, the contain/support/part relationships are a bit peculiar in that they’re not quite independent, they all represent a kind of location and thus if you change one you have to “reset” the others.
Actually, after playing with this a bit, perhaps this demonstrates some of the current weirdness you’re referring to?
There is a room. In it is a truck and a couch.
The truck is a vehicle.
The tailgate is part of the truck. It is an enterable supporter.
The couch is an enterable supporter.
Test me with "sit on couch / stand on couch / sit on tailgate / stand on tailgate / g"
You’d expect that the couch and the tailgate behave similarly with regard to sitting and standing, but they don’t.