sig
type elt
type t
val is_empty : t -> bool
val mem : t -> elt -> bool
val cardinal : t -> int
val iter : (elt -> unit) -> t -> unit
val fold : (elt -> 'a -> 'a) -> t -> 'a -> 'a
val elements : t -> elt list
val equal : t -> t -> bool
val empty : t
val add : t -> elt -> t
val remove : t -> elt -> t
val filter : (elt -> bool) -> t -> t
end