signature History =
sig
  type 'a history

  exception CANT_BACKUP_ANYMORE

  val new_history  : {obj:'a, limit:int} -> 'a history
  val apply        : ('a -> 'a) -> 'a history -> 'a history
  val set_limit    : 'a history -> int -> 'a history
  val initialValue : 'a history -> 'a
  val remove_past  : 'a history -> 'a history
  val project      : ('a -> 'b) -> 'a history -> 'b
  val undo         : 'a history -> 'a history
  val restore      : 'a history -> 'a history
  val save         : 'a history -> 'a history

end