Holding off on this until I see if the userland code gets out of hand or not, and if the solution is genericisable or not. It might be that a pre-made collection is less useful than just diy-ing exactly what you need each time.
Have tried building some use cases without this, there is not enough commonality to make this useful. Having a Dendriform<Map<number, Item>> where any adds increment a counter is enough.
Holding off on this until I see if the userland code gets out of hand or not, and if the solution is genericisable or not. It might be that a pre-made collection is less useful than just diy-ing exactly what you need each time.