Index: src/natools-s_expressions-generic_caches.ads
==================================================================
--- src/natools-s_expressions-generic_caches.ads
+++ src/natools-s_expressions-generic_caches.ads
@@ -1,7 +1,7 @@
 ------------------------------------------------------------------------------
--- Copyright (c) 2013-2014, Natacha Porté                                   --
+-- Copyright (c) 2013-2019, 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.        --
 --                                                                          --
@@ -94,10 +94,17 @@
    function Move (Source : in out S_Expressions.Descriptor'Class) return Cursor
      is (Move (Source).First);
       --  Return a cursor holding a copy of Original (which is
       --  destructively read)
 
+   function Conditional_Move
+     (Source : in out S_Expressions.Descriptor'Class)
+     return Cursor
+     is (if Source in Cursor then Cursor (Source) else Move (Source).First);
+      --  Return a copy of Source, with cheap copy if possible,
+      --  otherwise with destructive Move
+
 private
 
    type Atom_Access is access Atom;
    for Atom_Access'Storage_Pool use Atom_Pool;
    procedure Unchecked_Free is new Ada.Unchecked_Deallocation