Index: src/natools-constant_indefinite_ordered_maps.adb
==================================================================
--- src/natools-constant_indefinite_ordered_maps.adb
+++ src/natools-constant_indefinite_ordered_maps.adb
@@ -1,7 +1,7 @@
 ------------------------------------------------------------------------------
--- Copyright (c) 2014-2015, Natacha Porté                                   --
+-- Copyright (c) 2014-2017, 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.        --
 --                                                                          --
@@ -332,10 +332,19 @@
       Process.all
         (Accessor.Data.Nodes (Position.Index).Key.all,
          Accessor.Data.Nodes (Position.Index).Element.all);
    end Query_Element;
 
+
+   function Rank (Position : Cursor) return Ada.Containers.Count_Type is
+   begin
+      case Position.Is_Empty is
+         when True => return 0;
+         when False => return Position.Index;
+      end case;
+   end Rank;
+
 
 
    -----------------------------
    -- Non-Standard Operations --
    -----------------------------

Index: src/natools-constant_indefinite_ordered_maps.ads
==================================================================
--- src/natools-constant_indefinite_ordered_maps.ads
+++ src/natools-constant_indefinite_ordered_maps.ads
@@ -1,7 +1,7 @@
 ------------------------------------------------------------------------------
--- Copyright (c) 2014-2015, Natacha Porté                                   --
+-- Copyright (c) 2014-2017, 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.        --
 --                                                                          --
@@ -108,10 +108,14 @@
      with Pre => Has_Element (Left) or else raise Constraint_Error;
    function "<" (Left : Key_Type; Right : Cursor) return Boolean
      with Pre => Has_Element (Right) or else raise Constraint_Error;
    function ">" (Left : Key_Type; Right : Cursor) return Boolean
      with Pre => Has_Element (Right) or else raise Constraint_Error;
+
+   function Rank (Position : Cursor) return Ada.Containers.Count_Type;
+      --  Return 1-based numeric position of the element designated by Position
+      --  or zero when Position is empty.
 
 
    package Map_Iterator_Interfaces is new Ada.Iterator_Interfaces
      (Cursor, Has_Element);