keys with associated default values, so that find is no longer partial
find
module Key : sig ... end
val set : t ‑> 'a Key.t ‑> 'a ‑> t
val find : t ‑> 'a Key.t ‑> 'a
val change : t ‑> 'a Key.t ‑> f:('a ‑> 'a) ‑> t