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