diff --git a/src/core/Backtrackable_ref.ml b/src/core/Backtrackable_ref.ml
new file mode 100644
index 00000000..bc91cfd5
--- /dev/null
+++ b/src/core/Backtrackable_ref.ml
@@ -0,0 +1,29 @@
+
+type 'a t = {
+  mutable cur: 'a;
+  stack: 'a Vec.t;
+  copy: ('a -> 'a) option;
+}
+
+let create ?copy x: _ t =
+  {cur=x; stack=Vec.create(); copy}
+
+let[@inline] get self = self.cur
+let[@inline] set self x = self.cur <- x
+let[@inline] update self f = self.cur <- f self.cur
+
+let[@inline] n_levels self = Vec.size self.stack
+
+let[@inline] push_level self : unit =
+  let x = self.cur in
+  let x = match self.copy with None -> x | Some f -> f x in
+  Vec.push self.stack x
+
+let pop_levels self n : unit =
+  assert (n>=0);
+  if n > Vec.size self.stack then invalid_arg "Backtrackable_ref.pop_levels";
+  let i = Vec.size self.stack-n in
+  let x = Vec.get self.stack i in
+  self.cur <- x;
+  Vec.shrink self.stack i;
+  ()
diff --git a/src/core/Backtrackable_ref.mli b/src/core/Backtrackable_ref.mli
new file mode 100644
index 00000000..a1755115
--- /dev/null
+++ b/src/core/Backtrackable_ref.mli
@@ -0,0 +1,30 @@
+
+(** {1 Backtrackable ref} *)
+
+type 'a t
+
+val create : ?copy:('a -> 'a) -> 'a -> 'a t
+(** Create a backtrackable reference holding the given value initially.
+    @param copy if provided, will be used to copy the value when [push_level]
+    is called. *)
+
+val set : 'a t -> 'a -> unit
+(** Set the reference's current content *)
+
+val get : 'a t -> 'a
+(** Get the reference's current content *)
+
+val update : 'a t -> ('a -> 'a) -> unit
+(** Update the reference's current content *)
+  
+val push_level : _ t -> unit
+(** Push a backtracking level, copying the current value on top of some
+    stack. The [copy] function will be used if it was provided in {!create}. *)
+
+val n_levels : _ t -> int
+(** Number of saved values *)
+
+val pop_levels : _ t -> int -> unit
+(** Pop [n] levels, restoring to the value the reference was storing [n] calls
+    to [push_level] earlier.
+    @raise Invalid_argument if [n] is bigger than [n_levels]. *)
diff --git a/src/core/Msat.ml b/src/core/Msat.ml
index 1dbd99a3..be7f6ce4 100644
--- a/src/core/Msat.ml
+++ b/src/core/Msat.ml
@@ -49,6 +49,8 @@ module Make_mcsat = Solver.Make_mcsat
 module Make_cdcl_t = Solver.Make_cdcl_t
 module Make_pure_sat = Solver.Make_pure_sat
 
+module Backtrackable_ref = Backtrackable_ref
+
 (**/**)
 module Vec = Vec
 module Log = Log
diff --git a/src/sudoku/sudoku_solve.ml b/src/sudoku/sudoku_solve.ml
index 72dc1e6b..d69026eb 100644
--- a/src/sudoku/sudoku_solve.ml
+++ b/src/sudoku/sudoku_solve.ml
@@ -125,36 +125,7 @@ end = struct
     a
 end
 
-(** Backtrackable ref *)
-module B_ref : sig
-  type 'a t
-  val create : 'a -> 'a t
-  val set : 'a t -> 'a -> unit
-  val get : 'a t -> 'a
-  val update : 'a t -> ('a -> 'a) -> unit
-  val push_level : _ t -> unit
-  val pop_levels : _ t -> int -> unit
-end = struct
-  type 'a t = {
-    mutable cur: 'a;
-    stack: 'a Vec.t;
-  }
-
-  let create x: _ t = {cur=x; stack=Vec.create()}
-
-  let[@inline] get self = self.cur
-  let[@inline] set self x = self.cur <- x
-  let[@inline] update self f = self.cur <- f self.cur
-
-  let[@inline] push_level self : unit = Vec.push self.stack self.cur
-  let pop_levels self n : unit =
-    assert (n>=0 && n <= Vec.size self.stack);
-    let i = Vec.size self.stack-n in
-    let x = Vec.get self.stack i in
-    self.cur <- x;
-    Vec.shrink self.stack i;
-    ()
-end
+module B_ref = Msat.Backtrackable_ref
 
 module Solver : sig
   type t