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
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