diff --git a/Frap.v b/Frap.v
index a49b116..245a755 100644
--- a/Frap.v
+++ b/Frap.v
@@ -43,7 +43,7 @@ Ltac invert0 e := invert e; fail.
 Ltac invert1 e := invert0 e || (invert e; []).
 Ltac invert2 e := invert1 e || (invert e; [|]).
 
-Ltac simplify := repeat progress (simpl in *; intros; try autorewrite with core in *).
+Ltac simplify := repeat progress (simpl in *; intros; try autorewrite with core in *); repeat removeDups.
 Ltac propositional := intuition idtac.
 
 Ltac linear_arithmetic := intros;
diff --git a/ModelChecking.v b/ModelChecking.v
index cf8251c..94e18ce 100644
--- a/ModelChecking.v
+++ b/ModelChecking.v
@@ -190,9 +190,6 @@ Proof.
   apply oneStepClosure_split; simplify.
   invert H; simplify.
   apply singleton_in.
-  apply oneStepClosure_split; simplify.
-  invert H; simplify.
-  apply singleton_in.
   apply oneStepClosure_empty.
   simplify.
 
@@ -202,10 +199,55 @@ Proof.
   propositional; invert H0; try equality.
   invert H; equality.
   invert H1; equality.
-  invert H; equality.
-  invert H; try equality.
 
   simplify.
   propositional; subst; simplify; propositional.
               (* ^-- *)
 Qed.
+
+Hint Rewrite fact_init_is.
+
+Ltac model_check_done :=
+  apply MscDone; apply prove_oneStepClosure; simplify; propositional; subst;
+  repeat match goal with
+         | [ H : _ |- _ ] => invert H
+         end; simplify; equality.
+
+Ltac model_check_step :=
+  eapply MscStep; [
+    repeat ((apply oneStepClosure_empty; simplify)
+            || (apply oneStepClosure_split; [ simplify;
+                                              repeat match goal with
+                                                     | [ H : _ |- _ ] => invert H
+                                                     end; apply singleton_in | ]))
+  | simplify ].
+
+Ltac model_check_steps1 := model_check_done || model_check_step.
+Ltac model_check_steps := repeat model_check_steps1.
+
+Ltac model_check_finish := simplify; propositional; subst; simplify; equality.
+
+Ltac model_check_find_invariant :=
+  simplify; eapply invariantFor_weaken; [
+    apply multiStepClosure_ok; simplify; model_check_steps
+  | ].
+
+Ltac model_check := model_check_find_invariant; model_check_finish.
+
+Theorem factorial_ok_2_snazzy :
+  invariantFor (factorial_sys 2) (fact_correct 2).
+Proof.
+  model_check.
+Qed.
+
+Theorem factorial_ok_3 :
+  invariantFor (factorial_sys 3) (fact_correct 3).
+Proof.
+  model_check.
+Qed.
+
+Theorem factorial_ok_4 :
+  invariantFor (factorial_sys 4) (fact_correct 4).
+Proof.
+  model_check.
+Qed.
diff --git a/Sets.v b/Sets.v
index 2e7923b..bf91761 100644
--- a/Sets.v
+++ b/Sets.v
@@ -126,3 +126,52 @@ End properties.
 Hint Resolve subseteq_refl subseteq_In.
 
 Hint Rewrite union_constant.
+
+
+(** * Removing duplicates from constant sets *)
+
+Inductive removeDups A : list A -> list A -> Prop :=
+| RdNil : removeDups nil nil
+| RdNew : forall x ls ls',
+  ~List.In x ls
+  -> removeDups ls ls'
+  -> removeDups (x :: ls) (x :: ls')
+| RdDup : forall x ls ls',
+  List.In x ls
+  -> removeDups ls ls'
+  -> removeDups (x :: ls) ls'.
+
+Theorem removeDups_fwd : forall A x (ls ls' : list A),
+  removeDups ls ls'
+  -> List.In x ls
+  -> List.In x ls'.
+Proof.
+  induction 1; simpl; intuition.
+  subst; eauto.
+Qed.
+
+Theorem removeDups_bwd : forall A x (ls ls' : list A),
+  removeDups ls ls'
+  -> List.In x ls'
+  -> List.In x ls.
+Proof.
+  induction 1; simpl; intuition.
+Qed.
+
+Theorem removeDups_ok : forall A (ls ls' : list A),
+  removeDups ls ls'
+  -> constant ls = constant ls'.
+Proof.
+  intros.
+  apply sets_equal.
+  unfold constant; intuition eauto using removeDups_fwd, removeDups_bwd.
+Qed.
+
+Ltac removeDups :=
+  match goal with
+  | [ |- context[constant ?ls] ] =>
+    erewrite (@removeDups_ok _ ls)
+      by repeat (apply RdNil
+                 || (apply RdNew; [ simpl; intuition congruence | ])
+                 || (apply RdDup; [ simpl; intuition congruence | ]))
+  end.