sig
  type t = string
  val string : string -> Digest.t
  val substring : string -> pos:int -> len:int -> Digest.t
  external channel : Pervasives.in_channel -> len:int -> Digest.t
    = "md5_chan"
  val file : string -> Digest.t
  val output : Pervasives.out_channel -> Digest.t -> unit
  val input : Pervasives.in_channel -> Digest.t
end