diff --git a/src/bdd.ml b/src/bdd.ml
index 332e942..9b30377 100644
--- a/src/bdd.ml
+++ b/src/bdd.ml
@@ -37,30 +37,21 @@ module Hash = struct
   let hash b = b.tag
 end
 
-module Hash2 = struct
-  type t = hidden * hidden
-
-  let equal (x1, y1) (x2, y2) = x1 == x2 && y1 == y2
-
-  let hash (x, y) = x.tag + (19 * y.tag)
-end
-
 let hc = Hbdd.hashcons
 
 let view x = x.node
 
 module Mem = Memo.MakeWeak (Hash)
-module Mem2 = Memo.MakeWeak (Hash)
 
 let true_bdd = hc True
 
 let false_bdd = hc False
 
 let get_order bdd =
-  match view bdd with True | False -> max_int | Node (v, _, _) -> v
+  match view bdd with True | False -> -1 | Node (v, _, _) -> v
 
 let node v l h =
-  if v >= get_order l || v >= get_order h then invalid_arg "node" ;
+  if v <= get_order l || v <= get_order h then invalid_arg "node" ;
   if Hash.equal l h then l else hc (Node (v, l, h))
 
 let var_bdd v = node v false_bdd true_bdd
@@ -72,7 +63,14 @@ let rec fprintf fmt bdd =
   | False ->
       Format.fprintf fmt "false"
   | Node (v, l, h) ->
-      Format.fprintf fmt "%d ? (%a) : (%a)" v fprintf l fprintf h
+      Format.fprintf fmt "%d ? (%a) : (%a)" v fprintf h fprintf l
+
+let to_string bdd =
+  let buff = Buffer.create 512 in
+  let fmt = Format.formatter_of_buffer buff in
+  fprintf fmt bdd ;
+  Format.pp_print_flush fmt () ;
+  Buffer.contents buff
 
 let of_bool = function true -> true_bdd | false -> false_bdd
 
@@ -88,55 +86,53 @@ let neg x =
           node var (neg low) (neg high))
     x
 
-let comb_comm op x y =
-  Mem2.memo
-    (fun comb_comm n1 n2 ->
-      match (view n1, view n2) with
-      | Node (v1, l1, h1), Node (v2, l2, h2) when v1 = v2 ->
-          node v1 (comb_comm l1 l2) (comb_comm h1 h2)
-      | Node (v1, l1, h1), Node (v2, _, _) when v1 < v2 ->
-          node v1 (comb_comm l1 n2) (comb_comm h1 n2)
-      | Node (_, _, _), Node (v2, l2, h2) ->
-          node v2 (comb_comm n1 l2) (comb_comm n1 h2)
-      | True, Node (v, l, h) | Node (v, l, h), True ->
-          node v (comb_comm l true_bdd) (comb_comm h true_bdd)
-      | False, Node (v, l, h) | Node (v, l, h), False ->
-          node v (comb_comm l false_bdd) (comb_comm h false_bdd)
-      | False, False ->
-          of_bool (op false false)
-      | False, True | True, False ->
-          of_bool (op true false)
-      | True, True ->
-          of_bool (op true true))
-    x y
+(* TODO: memo 2 ? *)
+let rec comb_comm op n1 n2 =
+  let comb_comm = comb_comm op in
+  match (view n1, view n2) with
+  | Node (v1, l1, h1), Node (v2, l2, h2) when v1 = v2 ->
+      node v1 (comb_comm l1 l2) (comb_comm h1 h2)
+  | Node (v1, l1, h1), Node (v2, _, _) when v1 < v2 ->
+      node v1 (comb_comm l1 n2) (comb_comm h1 n2)
+  | Node (_, _, _), Node (v2, l2, h2) ->
+      node v2 (comb_comm n1 l2) (comb_comm n1 h2)
+  | True, Node (v, l, h) | Node (v, l, h), True ->
+      node v (comb_comm l true_bdd) (comb_comm h true_bdd)
+  | False, Node (v, l, h) | Node (v, l, h), False ->
+      node v (comb_comm l false_bdd) (comb_comm h false_bdd)
+  | False, False ->
+      of_bool (op false false)
+  | False, True | True, False ->
+      of_bool (op true false)
+  | True, True ->
+      of_bool (op true true)
 
-let comb op x y =
-  Mem2.memo
-    (fun comb n1 n2 ->
-      match (view n1, view n2) with
-      | Node (v1, l1, h1), Node (v2, l2, h2) when v1 = v2 ->
-          node v1 (comb l1 l2) (comb h1 h2)
-      | Node (v1, l1, h1), Node (v2, _, _) when v1 < v2 ->
-          node v1 (comb l1 n2) (comb h1 n2)
-      | Node (_, _, _), Node (v2, l2, h2) ->
-          node v2 (comb n1 l2) (comb n1 h2)
-      | True, Node (v, l, h) ->
-          node v (comb true_bdd l) (comb true_bdd h)
-      | Node (v, l, h), True ->
-          node v (comb l true_bdd) (comb h true_bdd)
-      | False, Node (v, l, h) ->
-          node v (comb false_bdd l) (comb false_bdd h)
-      | Node (v, l, h), False ->
-          node v (comb l false_bdd) (comb h false_bdd)
-      | False, False ->
-          of_bool (op false false)
-      | False, True ->
-          of_bool (op false true)
-      | True, False ->
-          of_bool (op true false)
-      | True, True ->
-          of_bool (op true true))
-    x y
+(* TODO: memo2 ? *)
+let rec comb op n1 n2 =
+  let comb = comb op in
+  match (view n1, view n2) with
+  | Node (v1, l1, h1), Node (v2, l2, h2) when v1 = v2 ->
+      node v1 (comb l1 l2) (comb h1 h2)
+  | Node (v1, l1, h1), Node (v2, _, _) when v1 < v2 ->
+      node v1 (comb l1 n2) (comb h1 n2)
+  | Node (_, _, _), Node (v2, l2, h2) ->
+      node v2 (comb n1 l2) (comb n1 h2)
+  | True, Node (v, l, h) ->
+      node v (comb true_bdd l) (comb true_bdd h)
+  | Node (v, l, h), True ->
+      node v (comb l true_bdd) (comb h true_bdd)
+  | False, Node (v, l, h) ->
+      node v (comb false_bdd l) (comb false_bdd h)
+  | Node (v, l, h), False ->
+      node v (comb l false_bdd) (comb h false_bdd)
+  | False, False ->
+      of_bool (op false false)
+  | False, True ->
+      of_bool (op false true)
+  | True, False ->
+      of_bool (op true false)
+  | True, True ->
+      of_bool (op true true)
 
 let conj = comb_comm (fun x y -> x && y)
 
@@ -156,7 +152,7 @@ let compute tbl =
     | Node (v, l, h) -> (
       match Hashtbl.find tbl v with
       | exception Not_found ->
-          node v (compute_aux h) (compute_aux l)
+          node v (compute_aux l) (compute_aux h)
       | true ->
           compute_aux h
       | false ->
@@ -188,10 +184,7 @@ let size =
 
 let is_sat bdd = match view bdd with False -> false | _ -> true
 
-let count_sat maxn =
-  let get_var bdd =
-    match view bdd with False | True -> maxn | Node (v, _, _) -> v
-  in
+let count_sat card =
   let count =
     Mem.memo (fun count bdd ->
         match view bdd with
@@ -200,10 +193,11 @@ let count_sat maxn =
         | True ->
             1
         | Node (v, l, h) ->
-            assert ((0 <= v && v < maxn) || (Printf.printf "v = %d\n" v ; false)) ;
-            (count l lsl (get_var l - v - 1)) + (count h lsl (get_var h - v - 1)))
+            assert (0 <= v && v < card) ;
+            let count_side s = count s lsl (v - get_order s - 1) in
+            count_side h + count_side l)
   in
-  fun bdd -> count bdd lsl get_var bdd
+  fun bdd -> count bdd lsl (card - get_order bdd - 1)
 
 let any_sat =
   let rec aux assign bdd =
diff --git a/test/dune b/test/dune
new file mode 100644
index 0000000..4f1ca19
--- /dev/null
+++ b/test/dune
@@ -0,0 +1,3 @@
+(test
+ (name test)
+ (libraries bdd))
diff --git a/test/test.ml b/test/test.ml
new file mode 100644
index 0000000..299d13f
--- /dev/null
+++ b/test/test.ml
@@ -0,0 +1,65 @@
+open Bdd
+
+let _ =
+  let order = get_order true_bdd in
+  assert (order = -1) ;
+  let order = get_order false_bdd in
+  assert (order = -1) ;
+  let one_var = var_bdd 0 in
+  assert (get_order one_var = 0) ;
+  assert (to_string one_var = "0 ? (true) : (false)") ;
+  assert (of_bool true == true_bdd) ;
+  assert (of_bool false == false_bdd) ;
+  assert (neg true_bdd == false_bdd) ;
+  assert (neg false_bdd == true_bdd) ;
+  assert (to_string (neg one_var) = "0 ? (false) : (true)") ;
+  let bdd = conj true_bdd true_bdd in
+  assert (bdd == true_bdd) ;
+  let bdd = disj false_bdd true_bdd in
+  assert (bdd == true_bdd) ;
+  let bdd = disj false_bdd one_var in
+  assert (bdd == one_var) ;
+  let bdd = imp false_bdd true_bdd in
+  assert (bdd == true_bdd) ;
+  let bdd = imp true_bdd false_bdd in
+  assert (bdd == false_bdd) ;
+  let bdd = eq false_bdd true_bdd in
+  assert (bdd == false_bdd) ;
+  let tbl = Hashtbl.create 32 in
+  Hashtbl.add tbl 0 true ;
+  assert (compute tbl one_var == true_bdd) ;
+  Hashtbl.replace tbl 0 false ;
+  assert (compute tbl one_var == false_bdd) ;
+  Hashtbl.clear tbl ;
+  assert (compute tbl one_var == one_var) ;
+  assert (size true_bdd = 0) ;
+  assert (size false_bdd = 0) ;
+  assert (size one_var = 1) ;
+  let one_var' = var_bdd 1 in
+  let bdd = node 2 one_var' one_var in
+  assert (size bdd = 2) ;
+  assert (is_sat true_bdd = true) ;
+  assert (is_sat false_bdd = false) ;
+  assert (is_sat bdd = true) ;
+  assert (count_sat 42 false_bdd = 0) ;
+  assert (count_sat 0 true_bdd = 1) ;
+  assert (count_sat 1 true_bdd = 2) ;
+  assert (count_sat 2 true_bdd = 4) ;
+  assert (count_sat 1 one_var = 1) ;
+  assert (count_sat 2 one_var' = 2) ;
+  assert (count_sat 3 bdd = 4) ;
+  assert (count_sat 4 bdd = 8) ;
+  assert (any_sat false_bdd = None) ;
+  assert (any_sat true_bdd = Some []) ;
+  assert (any_sat one_var = Some [(0, true)]) ;
+  ( match any_sat bdd with
+  | None ->
+      assert false
+  | Some assign ->
+      assert (
+        List.sort compare assign = [(0, true); (2, true)]
+        || List.sort compare assign = [(1, true); (2, false)] ) ) ;
+  assert (
+    List.sort compare (List.map (fun el -> List.sort compare el) (all_sat bdd))
+    = [[(0, true); (2, true)]; [(1, true); (2, false)]] ) ;
+  Format.printf "Tests are OK !@."