Meta.KeyMetadata keys.
val make : ?pp_value:(Stdlib.Format.formatter -> 'a -> unit) -> unit -> 'a tmake ~pp_value () is a new key using pp to format its values (defaults formats "<value>".
val id : 'a t -> 'a Stdlib.Type.Id.tid is the identifier of the key.
val pp_value : 'a t -> Stdlib.Format.formatter -> 'a -> unitpp_value key is key's value formatter.