IMPLEMENTATION MODULE DString;
(* Completes the opaque String here; clients see only the opaque type. *)
TYPE String = POINTER TO RECORD v : INTEGER END;
PROCEDURE Make(n : INTEGER) : String;
VAR s : String;
BEGIN NEW(s); s^.v := n; RETURN s END Make;
PROCEDURE Get(s : String) : INTEGER;
BEGIN RETURN s^.v END Get;
PROCEDURE IsNil(s : String) : BOOLEAN;
BEGIN RETURN s = NIL END IsNil;
END DString.