Extensible Map for State
This data structure maps keys to values of any type (although the type is fixed for a given key). Useful for configuration.
val create_key : unit -> 'a keyval add : 'a key -> 'a -> t -> tval get : 'a key -> t -> 'a optionval get_exn : 'a key -> t -> 'aval get_or : or_:'a -> 'a key -> t -> 'a