Index: src/natools-references.adb
==================================================================
--- src/natools-references.adb
+++ src/natools-references.adb
@@ -1,7 +1,7 @@
 ------------------------------------------------------------------------------
--- Copyright (c) 2013, Natacha Porté                                        --
+-- Copyright (c) 2013-2014, Natacha Porté                                   --
 --                                                                          --
 -- Permission to use, copy, modify, and distribute this software for any    --
 -- purpose with or without fee is hereby granted, provided that the above   --
 -- copyright notice and this permission notice appear in all copies.        --
 --                                                                          --
@@ -73,27 +73,32 @@
       Ref.Data := new Held_Data'(Constructor.all);
       Ref.Count := new Counter'(1);
    end Replace;
 
 
-   function Create
-     (Constructor : not null access function return Data_Access)
-      return Immutable_Reference is
+   function Create (Data : in Data_Access) return Immutable_Reference is
    begin
-      return (Ada.Finalization.Controlled with
-         Data => Constructor.all,
-         Count => new Counter'(1));
+      if Data = null then
+         return Null_Immutable_Reference;
+      else
+         return (Ada.Finalization.Controlled with
+            Data => Data,
+            Count => new Counter'(1));
+      end if;
    end Create;
 
 
    procedure Replace
      (Ref : in out Immutable_Reference;
-      Constructor : not null access function return Data_Access) is
+      Data : in Data_Access) is
    begin
       Finalize (Ref);
-      Ref.Data := Constructor.all;
-      Ref.Count := new Counter'(1);
+
+      if Data /= null then
+         Ref.Data := Data;
+         Ref.Count := new Counter'(1);
+      end if;
    end Replace;
 
 
    procedure Reset (Ref : in out Immutable_Reference) is
    begin

Index: src/natools-references.ads
==================================================================
--- src/natools-references.ads
+++ src/natools-references.ads
@@ -1,7 +1,7 @@
 ------------------------------------------------------------------------------
--- Copyright (c) 2013, Natacha Porté                                        --
+-- Copyright (c) 2013-2014, Natacha Porté                                   --
 --                                                                          --
 -- Permission to use, copy, modify, and distribute this software for any    --
 -- purpose with or without fee is hereby granted, provided that the above   --
 -- copyright notice and this permission notice appear in all copies.        --
 --                                                                          --
@@ -52,19 +52,19 @@
    procedure Replace
      (Ref : in out Immutable_Reference;
       Constructor : not null access function return Held_Data);
       --  Replace the object held in Ref with a newly created object
 
-   function Create
-     (Constructor : not null access function return Data_Access)
-      return Immutable_Reference;
-      --  Create a new held object and return a reference to it
-
-   procedure Replace
-     (Ref : in out Immutable_Reference;
-      Constructor : not null access function return Data_Access);
-      --  Replace the object held in Ref with a newly created object
+   function Create (Data : in Data_Access) return Immutable_Reference;
+      --  Create a new reference from Data.
+      --  From this point the referred object is owned by this
+      --  package and must NOT be freed or changed or accessed.
+
+   procedure Replace (Ref : in out Immutable_Reference; Data : in Data_Access);
+      --  Integrate Data into Ref.
+      --  From this point the referred object is owned by this
+      --  package and must NOT be freed or changed or accessed.
 
    procedure Reset (Ref : in out Immutable_Reference);
       --  Empty Ref
 
    function Is_Empty (Ref : Immutable_Reference) return Boolean;