diff --git a/LGPL/Makefile.in b/LGPL/Makefile.in
index 73d95d5be..fe6f9e033 100644
--- a/LGPL/Makefile.in
+++ b/LGPL/Makefile.in
@@ -28,6 +28,7 @@ PROGRAMS= $(srcdir)/base64.pl \
 	$(srcdir)/broadcast.pl \
 	$(srcdir)/debug.pl \
 	$(srcdir)/maplist.pl \
+	$(srcdir)/nb_set.pl \
 	$(srcdir)/operators.pl \
 	$(srcdir)/option.pl \
 	$(srcdir)/prolog_source.pl \
diff --git a/LGPL/nb_set.pl b/LGPL/nb_set.pl
new file mode 100644
index 000000000..2b3b57609
--- /dev/null
+++ b/LGPL/nb_set.pl
@@ -0,0 +1,124 @@
+/*  $Id$
+
+    Part of SWI-Prolog
+
+    Author:        Jan Wielemaker
+    E-mail:        jan@science.uva.nl
+    WWW:           http://www.swi-prolog.org
+    Copyright (C): 1985-2005, University of Amsterdam
+
+    This program is free software; you can redistribute it and/or
+    modify it under the terms of the GNU General Public License
+    as published by the Free Software Foundation; either version 2
+    of the License, or (at your option) any later version.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU Lesser General Public
+    License along with this library; if not, write to the Free Software
+    Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+
+    As a special exception, if you link this library with other files,
+    compiled with a Free Software compiler, to produce an executable, this
+    library does not by itself cause the resulting executable to be covered
+    by the GNU General Public License. This exception does not however
+    invalidate any other reasons why the executable file might be covered by
+    the GNU General Public License.
+*/
+
+:- module(nb_set,
+	  [ empty_nb_set/1,		% -EmptySet
+	    add_nb_set/2,		% +Key, !Set
+	    add_nb_set/3,		% +Key, !Set, ?New
+	    gen_nb_set/2,		% +Set, -Key
+	    size_nb_set/2,		% +Set, -Size
+	    nb_set_to_list/2		% +Set, -List
+	  ]).
+
+
+/** <module> Non-backtrackable sets
+
+This library provides a non-backtrackabe set. It is based on
+nb_setarg/3. See the SWI-Prolog manual for details.
+
+@author Jan Wielemaker
+@tbd	Base this work on AVL trees rather then unbalanced trees.
+*/
+
+		 /*******************************
+		 *    NON-BACKTRACKABLE SETS	*
+		 *******************************/
+
+%%	empty_nb_set(-Set)
+%
+%	Create an empty non-backtrackable set.
+
+empty_nb_set(nb_set(t)).
+
+%%	add_nb_set(+Key, !Set) is det.
+%%	add_nb_set(+Key, !Set, ?New) is semidet.
+%
+%	Insert an element into the set. If the element is already in the
+%	set, nothing happens. New is =true= if   Key  was added as a new
+%	element to the set and =false= otherwise.
+
+add_nb_set(Key, Set) :-
+	add_nb_set(Key, Set, _).
+add_nb_set(Key, Set, New) :-
+	(   empty_nb_set(Set)
+	->  New = true,
+	    nb_setarg(1, Set, t(Key, t, t))
+	;   arg(1, Set, Tree),
+	    '$btree_find_node'(Key, Tree, Node, Arg),
+	    (	Arg == 1
+	    ->	New = false
+	    ;	New = true,
+		nb_setarg(Arg, Node, t(Key, t, t))
+	    )
+	).
+
+
+%%	nb_set_to_list(+Set, -List)
+%
+%	Get the elements of a an nb_set. List is sorted to the standard
+%	order of terms.
+
+nb_set_to_list(nb_set(Set), List) :-
+	phrase(nb_set_to_list(Set), List).
+
+nb_set_to_list(t) -->
+	[].
+nb_set_to_list(t(Val, Left, Right)) -->
+	nb_set_to_list(Left),
+	[Val],
+	nb_set_to_list(Right).
+
+
+%%	gen_nb_set(+Set, -Key)
+%
+%	Enumerate the members of a set in the standard order of terms.
+
+gen_nb_set(nb_set(Tree), Key) :-
+	gen_set(Tree, Key).
+
+gen_set(t(Val, Left, Right), Key) :-
+	(   gen_set(Left, Key)
+	;   Key = Val
+	;   gen_set(Right, Key)
+	).
+
+%%	size_nb_set(+Set, -Size)
+%
+%	Unify Size with the number of elements in the set
+
+size_nb_set(nb_set(Tree), Size) :-
+	set_size(Tree, Size).
+
+set_size(t, 0).
+set_size(t(_,L,R), Size) :-
+	set_size(L, SL),
+	set_size(R, SR),
+	Size is SL+SR+1.