diff --git a/AbstractInterpretation.v b/AbstractInterpretation.v
index 8a2d71c..372e343 100644
--- a/AbstractInterpretation.v
+++ b/AbstractInterpretation.v
@@ -12,8 +12,8 @@ Record absint := {
   Typeof :> Set;
   (* This [:>] notation lets us treat any [absint] as its [Typeof],
    * automatically. *)
-  Top : Typeof;
-  (* The least precise element of the lattice *)
+  Const : nat -> Typeof;
+  (* Most accurate representation of a constant *)
   Join : Typeof -> Typeof -> Typeof;
   (* Least upper bound of two elements *)
   Represents : nat -> Typeof -> Prop
@@ -21,8 +21,8 @@ Record absint := {
 }.
 
 Definition absint_sound (a : absint) :=
-  (* [Top] really does cover everything. *)
-  (forall n, a.(Represents) n a.(Top))
+  (* [Const] gives accurate answers. *)
+  (forall n, a.(Represents) n (a.(Const) n))
 
   (* [Join] really does return an upper bound. *)
   /\ (forall x y n, a.(Represents) n x
@@ -33,12 +33,15 @@ Definition absint_sound (a : absint) :=
 Definition astate (a : absint) := fmap var a.
 Definition astates (a : absint) := fmap cmd (astate a).
 
+Definition compatible1 a (s : astate a) (v : valuation) (c : cmd) : Prop :=
+  forall x n xa, v $? x = Some n
+                 -> s $? x = Some xa
+                 -> a.(Represents) n xa.
+
 Inductive compatible a (ss : astates a) (v : valuation) (c : cmd) : Prop :=
 | Compatible : forall s,
   ss $? c = Some s
-  -> (forall x n xa, v $? x = Some n
-                     -> s $? x = Some xa
-                     -> a.(Represents) n xa)
+  -> compatible1 s v c
   -> compatible ss v c.
 
 Definition merge_astate a : astate a -> astate a -> astate a :=
@@ -68,6 +71,7 @@ Inductive oneStepClosure a : astates a -> astates a -> Prop :=
   oneStepClosure $0 $0
 | OscCons : forall ss c s ss' ss'',
   (forall v c' v', step (v, c) (v', c')
+    -> compatible1 s v c
     -> compatible ss' v' c')
   -> oneStepClosure ss ss''
   -> oneStepClosure (ss $+ (c, s)) (merge_astates ss' ss'').
@@ -95,6 +99,7 @@ Proof.
 
   econstructor.
   eassumption.
+  unfold compatible1.
   simplify.
   equality.
 
@@ -123,3 +128,273 @@ Proof.
   simplify.
   trivial.
 Qed.
+
+
+(** * Example: even-odd analysis *)
+
+Inductive parity := Even | Odd | Either.
+
+Definition isEven (n : nat) := exists k, n = k * 2.
+Definition isOdd (n : nat) := exists k, n = k * 2 + 1.
+
+Theorem decide_parity : forall n, isEven n \/ isOdd n.
+Proof.
+  induct n; simplify; propositional.
+
+  left; exists 0; linear_arithmetic.
+
+  invert H.
+  right.
+  exists x; linear_arithmetic.
+
+  invert H.
+  left.
+  exists (x + 1); linear_arithmetic.
+Qed.
+  
+Theorem notEven_odd : forall n, ~isEven n -> isOdd n.
+Proof.
+  simplify.
+  assert (isEven n \/ isOdd n).
+  apply decide_parity.
+  propositional.
+Qed.  
+
+Theorem isEven_0 : isEven 0.
+Proof.
+  exists 0; linear_arithmetic.
+Qed.
+
+Theorem isEven_1 : ~isEven 1.
+Proof.
+  propositional; invert H; linear_arithmetic.
+Qed.
+
+Theorem isEven_S_Even : forall n, isEven n -> ~isEven (S n).
+Proof.
+  propositional; invert H; invert H0; linear_arithmetic.
+Qed.
+
+Theorem isEven_S_Odd : forall n, ~isEven n -> isEven (S n).
+Proof.
+  propositional.
+  apply notEven_odd in H.
+  invert H.
+  exists (x + 1); linear_arithmetic.
+Qed.
+
+Hint Resolve isEven_0 isEven_1 isEven_S_Even isEven_S_Odd.  
+
+Definition parity_flip (p : parity) :=
+  match p with
+  | Even => Odd
+  | Odd => Even
+  | Either => Either
+  end.
+
+Fixpoint parity_const (n : nat) :=
+  match n with
+  | O => Even
+  | S n' => parity_flip (parity_const n')
+  end.
+
+Definition parity_join (x y : parity) :=
+  match x, y with
+  | Even, Even => Even
+  | Odd, Odd => Odd
+  | _, _ => Either
+  end.
+
+Inductive parity_rep : nat -> parity -> Prop :=
+| PrEven : forall n,
+  isEven n
+  -> parity_rep n Even
+| PrOdd : forall n,
+  ~isEven n
+  -> parity_rep n Odd
+| PrEither : forall n,
+  parity_rep n Either.
+
+Hint Constructors parity_rep.
+
+Definition parity_absint := {|
+  Const := parity_const;
+  Join := parity_join;
+  Represents := parity_rep
+|}.
+
+Lemma parity_const_sound : forall n,
+  parity_rep n (parity_const n).
+Proof.
+  induct n; simplify; eauto.
+  cases (parity_const n); simplify; eauto.
+  invert IHn; eauto.
+  invert IHn; eauto.
+Qed.
+
+Hint Resolve parity_const_sound.
+
+Theorem parity_sound : absint_sound parity_absint.
+Proof.
+  unfold absint_sound; propositional.
+
+  simplify; eauto.
+
+  invert H; cases y; simplify; eauto.
+
+  invert H; cases x; simplify; eauto.
+Qed.
+
+Definition loopy :=
+  "n" <- 100;;
+  "a" <- 0;;
+  while "n" loop
+    "a" <- "a" + "n";;
+    "n" <- "n" - 2
+  done.
+
+Theorem compatible_skip : forall (s : astate parity_absint) v c c' m,
+  compatible1 s v c
+  -> compatible (m $+ (c', s)) v c'.
+Proof.
+  unfold compatible1; simplify.
+  econstructor.
+  simplify; equality.
+  auto.
+Qed.
+
+Theorem compatible_skip2 : forall (s : astate parity_absint) v c c' m c'' s',
+  compatible1 s v c
+  -> c'' <> c'
+  -> compatible (m $+ (c', s) $+ (c'', s')) v c'.
+Proof.
+  unfold compatible1; simplify.
+  econstructor.
+  simplify; equality.
+  auto.
+Qed.
+
+Theorem compatible_const : forall (s : astate parity_absint) v c c' x n,
+  compatible1 s v c
+  -> compatible ($0 $+ (c', s $+ (x, parity_const n))) (v $+ (x, n)) c'.
+Proof.
+  unfold compatible1; simplify.
+  econstructor.
+  simplify; equality.
+  unfold compatible1.
+  simplify.
+  cases (x ==v x0); simplify.
+  invert H1.
+  invert H0.
+  eauto.
+
+  eapply H.
+  eassumption.
+  assumption.
+Qed.
+
+Hint Rewrite merge_empty1 merge_empty2 using solve [ eauto 1 ].
+Hint Rewrite merge_empty1_alt merge_empty2_alt using congruence.
+
+Lemma merge_astates_fok : forall x : option (astate parity_absint),
+  match x with Some x' => Some x' | None => None end = x.
+Proof.
+  simplify; cases x; equality.
+Qed.
+
+Lemma merge_astates_fok2 : forall x (y : option (astate parity_absint)),
+    match y with
+    | Some y' => Some (merge_astate x y')
+    | None => Some x
+    end = None -> False.
+Proof.
+  simplify; cases y; equality.
+Qed.
+
+Hint Resolve merge_astates_fok merge_astates_fok2.
+
+Hint Rewrite merge_add1 using solve [ eauto | unfold Sets.In; simplify; equality ].
+Hint Rewrite merge_add1_alt using solve [ congruence | unfold Sets.In; simplify; equality ].
+
+Ltac inList x xs :=
+  match xs with
+  | (x, _) => constr:true
+  | (_, ?xs') => inList x xs'
+  | _ => constr:false
+  end.
+
+Ltac maybe_simplify_map m found kont :=
+  match m with
+  | @empty ?A ?B => kont (@empty A B)
+  | ?m' $+ (?k, ?v) =>
+    let iL := inList k found in
+    match iL with
+    | true => maybe_simplify_map m' found kont
+    | false =>
+      maybe_simplify_map m' (k, found) ltac:(fun m' => kont (m' $+ (k, v)))
+    end
+  end.
+
+Ltac simplify_map' m found kont :=
+  match m with
+  | ?m' $+ (?k, ?v) =>
+    let iL := inList k found in
+      match iL with
+      | true => maybe_simplify_map m' found kont
+      | false =>
+        simplify_map' m' (k, found) ltac:(fun m' => kont (m' $+ (k, v)))
+      end
+  end.
+
+Ltac simplify_map :=
+  match goal with
+  | [ |- context[?m $+ (?k, ?v)] ] =>
+    simplify_map' (m $+ (k, v)) tt ltac:(fun m' =>
+                                           replace (m $+ (k, v)) with m' by maps_equal)
+  end.
+
+Definition easy :=
+  "n" <- 10;;
+  while "n" loop
+    "n" <- "n" - 2
+  done.
+
+Theorem easy_even : forall v n,
+  isEven n
+  -> invariantFor (trsys_of v easy) (fun p => exists n, fst p $? "n" = Some n /\ isEven n).
+Proof.
+  simplify.
+  eapply invariant_weaken.
+
+  unfold easy.
+  apply interpret_sound.
+
+  Ltac interpret1 :=
+    eapply InterpretStep; [ (repeat (apply OscNil || eapply OscCons);
+                              invert 1; repeat (match goal with
+                                                | [ H : step _ _ |- _ ] => invert H
+                                                | [ _ : match ?E with
+                                                        | Some n => n
+                                                        | None => 0
+                                                        end = 0 |- _ ] => cases E; try linear_arithmetic
+                                                end; simplify); simplify;
+                              try solve [ eapply compatible_skip; eassumption
+                                        | eapply compatible_skip2; eassumption || congruence
+                                        | eapply compatible_const; eassumption ]) | ];
+    repeat match goal with
+           | [ |- context[@add ?A ?B ?m _ _] ] =>
+             match m with
+             | @empty _ _ => fail 1
+             | _ => unify m (@empty A B)
+             end
+           end.
+
+  interpret1.
+  unfold merge_astates, merge_astate; simplify.
+  interpret1.
+  unfold merge_astates, merge_astate; simplify.
+  simplify_map.
+  interpret1.
+  unfold merge_astates, merge_astate; simplify.
+  simplify_map.
+  interpret1.
diff --git a/Map.v b/Map.v
index ae7f550..315dee7 100644
--- a/Map.v
+++ b/Map.v
@@ -71,6 +71,47 @@ Module Type S.
   Axiom lookup_merge : forall A B f (m1 m2 : fmap A B) k,
     merge f m1 m2 $? k = f (m1 $? k) (m2 $? k).
 
+  Axiom merge_empty1 : forall A B f (m : fmap A B),
+    (forall x, f None x = x)
+    -> merge f (@empty _ _) m = m.
+
+  Axiom merge_empty2 : forall A B f (m : fmap A B),
+    (forall x, f x None = x)
+    -> merge f m (@empty _ _) = m.
+
+  Axiom merge_empty1_alt : forall A B f (m : fmap A B),
+    (forall x, f None x = None)
+    -> merge f (@empty _ _) m = @empty _ _.
+
+  Axiom merge_empty2_alt : forall A B f (m : fmap A B),
+    (forall x, f x None = None)
+    -> merge f m (@empty _ _) = @empty _ _.
+
+  Axiom merge_add1 : forall A B f (m1 m2 : fmap A B) k v,
+    (forall x y, f (Some x) y = None -> False)
+    -> ~k \in dom m1
+    -> merge f (add m1 k v) m2 = match f (Some v) (lookup m2 k) with
+                                 | None => merge f m1 m2
+                                 | Some v => add (merge f m1 m2) k v
+                                 end.
+
+  Axiom merge_add2 : forall A B f (m1 m2 : fmap A B) k v,
+    (forall x y, f x (Some y) = None -> False)
+    -> ~k \in dom m2
+    -> merge f m1 (add m2 k v) = match f (lookup m1 k) (Some v) with
+                                 | None => merge f m1 m2
+                                 | Some v => add (merge f m1 m2) k v
+                                 end.
+
+  Axiom merge_add1_alt : forall A B f (m1 m2 : fmap A B) k v,
+    (forall x y, f (Some x) (Some y) = None -> False)
+    -> ~k \in dom m1
+    -> k \in dom m2
+    -> merge f (add m1 k v) m2 = match f (Some v) (lookup m2 k) with
+                                 | None => merge f m1 m2
+                                 | Some v => add (merge f m1 m2) k v
+                                 end.
+
   Axiom empty_includes : forall A B (m : fmap A B), empty A B $<= m.
 
   Axiom dom_empty : forall A B, dom (empty A B) = {}.
@@ -87,6 +128,8 @@ Module Type S.
 
   Hint Rewrite lookup_empty lookup_add_eq lookup_add_ne lookup_remove_eq lookup_remove_ne lookup_merge using congruence.
 
+  Hint Rewrite dom_empty dom_add.
+
   Ltac maps_equal :=
     apply fmap_ext; intros;
       repeat (subst; autorewrite with core; try reflexivity;
@@ -237,6 +280,95 @@ Module M : S.
     auto.
   Qed.
 
+  Theorem merge_empty1 : forall A B f (m : fmap A B),
+    (forall x, f None x = x)
+    -> merge f (@empty _ _) m = m.
+  Proof.
+    intros; apply fmap_ext; unfold lookup, merge; auto.
+  Qed.
+
+  Theorem merge_empty2 : forall A B f (m : fmap A B),
+    (forall x, f x None = x)
+    -> merge f m (@empty _ _) = m.
+  Proof.
+    intros; apply fmap_ext; unfold lookup, merge; auto.
+  Qed.
+
+  Theorem merge_empty1_alt : forall A B f (m : fmap A B),
+    (forall x, f None x = None)
+    -> merge f (@empty _ _) m = @empty _ _.
+  Proof.
+    intros; apply fmap_ext; unfold lookup, merge; auto.
+  Qed.
+
+  Theorem merge_empty2_alt : forall A B f (m : fmap A B),
+    (forall x, f x None = None)
+    -> merge f m (@empty _ _) = @empty _ _.
+  Proof.
+    intros; apply fmap_ext; unfold lookup, merge; auto.
+  Qed.
+
+  Theorem merge_add1 : forall A B f (m1 m2 : fmap A B) k v,
+    (forall x y, f (Some x) y = None -> False)
+    -> ~k \in dom m1
+    -> merge f (add m1 k v) m2 = match f (Some v) (lookup m2 k) with
+                                 | None => merge f m1 m2
+                                 | Some v => add (merge f m1 m2) k v
+                                 end.
+  Proof.
+    intros; apply fmap_ext; unfold lookup, merge, add; intros.
+    destruct (decide (k0 = k)); auto; subst.
+    case_eq (f (Some v) (m2 k)); intros.
+    case_eq (decide (k = k)); congruence.
+    exfalso; eauto.
+
+    case_eq (f (Some v) (m2 k)); intros.
+    destruct (decide (k0 = k)); congruence.
+    auto.
+  Qed.
+
+  Theorem merge_add2 : forall A B f (m1 m2 : fmap A B) k v,
+    (forall x y, f x (Some y) = None -> False)
+    -> ~k \in dom m2
+    -> merge f m1 (add m2 k v) = match f (lookup m1 k) (Some v) with
+                                 | None => merge f m1 m2
+                                 | Some v => add (merge f m1 m2) k v
+                                 end.
+  Proof.
+    intros; apply fmap_ext; unfold lookup, merge, add; intros.
+    destruct (decide (k0 = k)); auto; subst.
+    case_eq (f (m1 k) (Some v)); intros.
+    case_eq (decide (k = k)); congruence.
+    exfalso; eauto.
+
+    case_eq (f (m1 k) (Some v)); intros.
+    destruct (decide (k0 = k)); congruence.
+    auto.
+  Qed.
+
+  Theorem merge_add1_alt : forall A B f (m1 m2 : fmap A B) k v,
+    (forall x y, f (Some x) (Some y) = None -> False)
+    -> ~k \in dom m1
+    -> k \in dom m2
+    -> merge f (add m1 k v) m2 = match f (Some v) (lookup m2 k) with
+                                 | None => merge f m1 m2
+                                 | Some v => add (merge f m1 m2) k v
+                                 end.
+  Proof.
+    intros; apply fmap_ext; unfold lookup, merge, add; intros.
+    destruct (decide (k0 = k)); auto; subst.
+    case_eq (f (Some v) (m2 k)); intros.
+    case_eq (decide (k = k)); congruence.
+    case_eq (m2 k); intros.
+    rewrite H3 in H2.
+    exfalso; eauto.
+    congruence.
+
+    case_eq (f (Some v) (m2 k)); intros.
+    destruct (decide (k0 = k)); congruence.
+    auto.
+  Qed.
+
   Theorem empty_includes : forall A B (m : fmap A B), includes (empty (A := A) B) m.
   Proof.
     unfold includes, empty; intuition congruence.