OCaml library : Weak.S

`sig
type data
type t
val create : int -> Weak.S.t
val clear : Weak.S.t -> unit
val merge : Weak.S.t -> Weak.S.data -> Weak.S.data
val add : Weak.S.t -> Weak.S.data -> unit
val remove : Weak.S.t -> Weak.S.data -> unit
val find : Weak.S.t -> Weak.S.data -> Weak.S.data
val find_opt : Weak.S.t -> Weak.S.data -> Weak.S.data option
val find_all : Weak.S.t -> Weak.S.data -> Weak.S.data list
val mem : Weak.S.t -> Weak.S.data -> bool
val iter : (Weak.S.data -> unit) -> Weak.S.t -> unit
val fold : (Weak.S.data -> 'acc -> 'acc) -> Weak.S.t -> 'acc -> 'acc
val count : Weak.S.t -> int
val stats : Weak.S.t -> int * int * int * int * int * int
end`