`Model_summary` allows you to make the `Model` be mutable, but persist the necessary information to discover edge transitions in `on_display`. The old model is persisted as a summary after stabilization and is passed to the next invocation of `on_display`

`type t`````
`val create : Model.t ‑> t`

`create` extracts the information needed to compute edge transitions in `on_display`. If `Model.t` is immutable, this can be `Fn.id`. If you don't need to trigger external actions, then this can be `Fn.const ()`. If your model is mutable and you need to trigger actions, then this will need to be somewhere in between