Index: src/natools-s_expressions-atom_buffers.adb
==================================================================
--- src/natools-s_expressions-atom_buffers.adb
+++ src/natools-s_expressions-atom_buffers.adb
@@ -120,10 +120,16 @@
    function Length (Buffer : Atom_Buffer) return Count is
    begin
       return Buffer.Used;
    end Length;
 
+
+   function Capacity (Buffer : Atom_Buffer) return Count is
+   begin
+      return Buffer.Available;
+   end Capacity;
+
 
    function Data (Buffer : Atom_Buffer) return Atom is
    begin
       if Buffer.Ref.Is_Empty then
          pragma Assert (Buffer.Available = 0 and Buffer.Used = 0);

Index: src/natools-s_expressions-atom_buffers.ads
==================================================================
--- src/natools-s_expressions-atom_buffers.ads
+++ src/natools-s_expressions-atom_buffers.ads
@@ -43,10 +43,11 @@
 
    procedure Invert (Buffer : in out Atom_Buffer);
       --  Invert the order of bytes (first becomes last, etc)
 
    function Length (Buffer : Atom_Buffer) return Count;
+   function Capacity (Buffer : Atom_Buffer) return Count;
    function Data (Buffer : Atom_Buffer) return Atom;
    procedure Query
      (Buffer : in Atom_Buffer;
       Process : not null access procedure (Data : in Atom));
    procedure Read