`sig
type elt = string
type t
val empty : t
val is_empty : t -> bool
val mem : elt -> t -> bool
val add : elt -> t -> t
val singleton : elt -> t
val remove : elt -> t -> t
val union : t -> t -> t
val inter : t -> t -> t
val disjoint : t -> t -> bool
val diff : t -> t -> t
val compare : t -> t -> int
val equal : t -> t -> bool
val subset : t -> t -> bool
val iter : (elt -> unit) -> t -> unit
val map : (elt -> elt) -> t -> t
val fold : (elt -> 'a -> 'a) -> t -> 'a -> 'a
val for_all : (elt -> bool) -> t -> bool
val exists : (elt -> bool) -> t -> bool
val filter : (elt -> bool) -> t -> t
val filter_map : (elt -> elt option) -> t -> t
val partition : (elt -> bool) -> t -> t * t
val cardinal : t -> int
val elements : t -> elt list
val min_elt : t -> elt
val min_elt_opt : t -> elt option
val max_elt : t -> elt
val max_elt_opt : t -> elt option
val choose : t -> elt
val choose_opt : t -> elt option
val split : elt -> t -> t * bool * t
val find : elt -> t -> elt
val find_opt : elt -> t -> elt option
val find_first : (elt -> bool) -> t -> elt
val find_first_opt : (elt -> bool) -> t -> elt option
val find_last : (elt -> bool) -> t -> elt
val find_last_opt : (elt -> bool) -> t -> elt option
val of_list : elt list -> t
val to_seq_from : elt -> t -> elt Seq.t
val to_seq : t -> elt Seq.t
val to_rev_seq : t -> elt Seq.t
val add_seq : elt Seq.t -> t -> t
val of_seq : elt Seq.t -> t
end`