ADDED   src/natools-s_expressions-special_descriptors.adb
Index: src/natools-s_expressions-special_descriptors.adb
==================================================================
--- src/natools-s_expressions-special_descriptors.adb
+++ src/natools-s_expressions-special_descriptors.adb
@@ -0,0 +1,96 @@
+------------------------------------------------------------------------------
+-- Copyright (c) 2015, 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.        --
+--                                                                          --
+-- THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES --
+-- WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF         --
+-- MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR  --
+-- ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES   --
+-- WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN    --
+-- ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF  --
+-- OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.           --
+------------------------------------------------------------------------------
+
+package body Natools.S_Expressions.Special_Descriptors is
+
+   overriding function Current_Event
+     (Object : in Constant_Descriptor) return Events.Event is
+   begin
+      return Object.Event;
+   end Current_Event;
+
+
+   overriding function Current_Atom (Object : in Constant_Descriptor)
+     return Atom
+   is
+      pragma Unreferenced (Object);
+   begin
+      raise Program_Error with "Current_Atom called on a special descriptor";
+      return Null_Atom;
+   end Current_Atom;
+
+
+   overriding function Current_Level (Object : in Constant_Descriptor)
+     return Natural
+   is
+      pragma Unreferenced (Object);
+   begin
+      raise Program_Error with "Current_Level called on a special descriptor";
+      return 0;
+   end Current_Level;
+
+   overriding procedure Query_Atom
+     (Object : in Constant_Descriptor;
+      Process : not null access procedure (Data : in Atom))
+   is
+      pragma Unreferenced (Object, Process);
+   begin
+      raise Program_Error with "Query_Atom called on a special descriptor";
+   end Query_Atom;
+
+
+   overriding procedure Read_Atom
+     (Object : in Constant_Descriptor;
+      Data : out Atom;
+      Length : out Count)
+   is
+      pragma Unreferenced (Object, Data, Length);
+   begin
+      raise Program_Error with "Read_Atom called on a special descriptor";
+   end Read_Atom;
+
+
+   overriding procedure Next
+     (Object : in out Constant_Descriptor;
+      Event : out Events.Event)
+   is
+      pragma Unmodified (Object);
+   begin
+      Event := Object.Event;
+   end Next;
+
+
+   overriding procedure Lock
+     (Object : in out Constant_Descriptor;
+      State : out Lockable.Lock_State)
+   is
+      pragma Unreferenced (Object, State);
+   begin
+      raise Program_Error with "Lock called on a special descriptor";
+   end Lock;
+
+
+   overriding procedure Unlock
+     (Object : in out Constant_Descriptor;
+      State : in out Lockable.Lock_State;
+      Finish : in Boolean := True)
+   is
+      pragma Unreferenced (Object, State, Finish);
+   begin
+      raise Program_Error with "Unlock called on a special descriptor";
+   end Unlock;
+
+end Natools.S_Expressions.Special_Descriptors;

ADDED   src/natools-s_expressions-special_descriptors.ads
Index: src/natools-s_expressions-special_descriptors.ads
==================================================================
--- src/natools-s_expressions-special_descriptors.ads
+++ src/natools-s_expressions-special_descriptors.ads
@@ -0,0 +1,70 @@
+------------------------------------------------------------------------------
+-- Copyright (c) 2015, 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.        --
+--                                                                          --
+-- THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES --
+-- WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF         --
+-- MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR  --
+-- ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES   --
+-- WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN    --
+-- ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF  --
+-- OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.           --
+------------------------------------------------------------------------------
+
+------------------------------------------------------------------------------
+-- Natools.S_Expressions.Special_Descriptors provides descriptors blocked   --
+-- in End_Of_Input or Error states.                                         --
+-- The global variables should be task-safe, since there is no actual       --
+-- change in any of the primitives.                                         --
+------------------------------------------------------------------------------
+
+with Natools.S_Expressions.Lockable;
+
+package Natools.S_Expressions.Special_Descriptors is
+   pragma Preelaborate;
+
+   subtype Control_Event is Events.Event with Static_Predicate
+     => Control_Event in Events.Error | Events.End_Of_Input;
+
+   type Constant_Descriptor (Event : Control_Event) is new Lockable.Descriptor
+     with null record;
+
+   overriding function Current_Event (Object : in Constant_Descriptor)
+     return Events.Event;
+
+   overriding function Current_Atom (Object : in Constant_Descriptor)
+     return Atom;
+
+   overriding function Current_Level (Object : in Constant_Descriptor)
+     return Natural;
+
+   overriding procedure Query_Atom
+     (Object : in Constant_Descriptor;
+      Process : not null access procedure (Data : in Atom));
+
+   overriding procedure Read_Atom
+     (Object : in Constant_Descriptor;
+      Data : out Atom;
+      Length : out Count);
+
+   overriding procedure Next
+     (Object : in out Constant_Descriptor;
+      Event : out Events.Event);
+
+   overriding procedure Lock
+     (Object : in out Constant_Descriptor;
+      State : out Lockable.Lock_State);
+
+   overriding procedure Unlock
+     (Object : in out Constant_Descriptor;
+      State : in out Lockable.Lock_State;
+      Finish : in Boolean := True);
+
+
+   Empty_Descriptor : Constant_Descriptor (Events.End_Of_Input);
+   Error_Descriptor : Constant_Descriptor (Events.Error);
+
+end Natools.S_Expressions.Special_Descriptors;