Index: src/natools-s_expressions-lockable.adb
==================================================================
--- src/natools-s_expressions-lockable.adb
+++ src/natools-s_expressions-lockable.adb
@@ -51,10 +51,16 @@
    function Current_Level (Stack : Lock_Stack) return Natural is
    begin
       return Stack.Level;
    end Current_Level;
 
+
+   function Null_State return Lock_State is
+   begin
+      return (0, 0);
+   end Null_State;
+
 
 
    -------------------------------------
    -- Lockable Wrapper Implementation --
    -------------------------------------

Index: src/natools-s_expressions-lockable.ads
==================================================================
--- src/natools-s_expressions-lockable.ads
+++ src/natools-s_expressions-lockable.ads
@@ -46,10 +46,13 @@
       --  valid level in the stack, and if Allow_Gap is True and more than
       --  one item would be removed.
 
    function Current_Level (Stack : Lock_Stack) return Natural;
       --  Return the value on top of the stack
+
+   function Null_State return Lock_State;
+      --  Return an invalid Lock_State
 
 
    type Descriptor is limited interface and S_Expressions.Descriptor;
 
    procedure Lock