INTERFACE FileReWr;
IMPORT Wr, Pathname;
TYPE
  T <: Wr.T;
PROCEDURE Open(p: Pathname.T): T;

  (* If the file exists and the contents match what is written,
     then don't touch the file. Otherwise write the file. *)

  (* hint: Don't forget to call Wr.Close when you're done *)

  (* must not use in multiple threads *)

END FileReWr.