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+(* Coq 8.19.2 *)
+
+Require Import Arith Lia List String.
+
+Inductive Expr : Type :=
+| Arg: Expr
+| Num: nat -> Expr
+| Boolean: bool -> Expr
+| Add: Expr -> Expr -> Expr
+| LessThan: Expr -> Expr -> Expr
+(* Tuples *)
+| Single: Expr -> Expr
+| Concat: Expr -> Expr -> Expr
+| Get : Expr -> nat -> Expr
+(* Effects *)
+| Print : Expr -> Expr -> Expr
+| Store : Expr -> Expr -> Expr
+| Load : Expr -> Expr
+(* pred in then else *)
+| If : Expr -> Expr -> Expr -> Expr -> Expr
+(* in pred out *)
+| DoWhile : Expr -> Expr -> Expr -> Expr
+| Ctx : Assumption -> Expr -> Expr
+with Assumption : Type :=
+| InThen : Expr -> Expr -> Assumption
+| InElse : Expr -> Expr -> Assumption
+| InDoWhile : Expr -> Expr -> Expr -> Assumption
+.
+
+Inductive Typ : Type :=
+| TInt: Typ
+| TState: Typ
+| TTuple: list Type -> Typ.
+
+Record State := 
+{
+  (* For now, only nats can be stored
+    addr -> option nat *)
+  Mem: list (nat * option nat);
+  Log: list nat
+}.
+
+Inductive Value : Type :=
+| VNum: nat -> Value
+| VBoolean: bool -> Value
+| VTuple: list Value -> Value
+| VState: State -> Value.
+
+Definition initstate :=
+  (VState {| Mem := nil; Log := nil |}).
+
+Inductive nth_element {A : Type} : nat -> list A -> A -> Prop :=
+| nth_zero : forall x l, nth_element 0 (x :: l) x
+| nth_S : forall n x y l, nth_element n l x -> nth_element (S n) (y :: l) x.
+
+(* Big-step judgement *)
+Inductive BS : Value -> Expr -> Value -> Prop :=
+| EArg : forall a,
+  BS a Arg a
+| ENum : forall a n,
+  BS a (Num n) (VNum n)
+| EBoolean : forall a b,
+  BS a (Boolean b) (VBoolean b)
+| EAdd : forall a e1 e2 n1 n2,
+  BS a e1 (VNum n1) ->
+  BS a e2 (VNum n2) ->
+  BS a (Add e1 e2) (VNum (n1 + n2))
+| ELessThan : forall a e1 e2 n1 n2,
+  BS a e1 (VNum n1) ->
+  BS a e2 (VNum n2) ->
+  BS a (LessThan e1 e2) (VBoolean (Nat.ltb n1 n2))
+| ESingle : forall a e v,
+  BS a e v ->
+  BS a (Single e) (VTuple (cons v nil))
+| EConcat: forall a e1 e2 t1 t2,
+  BS a e1 (VTuple t1) ->
+  BS a e2 (VTuple t2) ->
+  BS a (Concat e1 e2) (VTuple (t1 ++ t2))
+| EGet: forall a e vs i v,
+  BS a e (VTuple vs) ->
+  nth_element i vs v ->
+  BS a (Get e i) v
+| EPrint : forall a estate mem log e n,
+  BS a estate (VState {| Mem := mem; Log := log |}) ->
+  BS a e (VNum n) ->
+  BS a (Print estate e) (VState {| Mem := mem; Log := cons n log|})
+| EIfTrue : forall a ep ei et ee vi vt,
+  BS a ep (VBoolean true) ->
+  BS a ei vi ->
+  BS vi et vt ->
+  BS a (If ep ei et ee) vt
+| EIfFalse : forall a ep ei et ee vi ve,
+  BS a ep (VBoolean false) ->
+  BS a ei vi ->
+  BS vi ee ve ->
+  BS a (If ep ei et ee) ve
+| EDoWhileTrue : forall a ei ep eo vi vo vfin,
+  BS a ei vi ->
+  BS vi ep (VBoolean true) ->
+  BS vi eo vo ->
+  BS vo (DoWhile Arg ep eo) vfin ->
+  BS a (DoWhile ei ep eo) vfin
+| EDoWhileFalse : forall a ei ep eo vi vo,
+  BS a ei vi ->
+  BS vi ep (VBoolean false) ->
+  BS vi eo vo ->
+  BS a (DoWhile ei ep eo) vo
+| ECtxInThen : forall a ei ep e v a0,
+  BS a e v ->
+  BS a0 ei a ->
+  BS a0 ep (VBoolean true) ->
+  BS a (Ctx (InThen ep ei) e) v
+| ECtxInElse : forall a ei ep e v a0,
+  BS a e v ->
+  BS a0 ei a ->
+  BS a0 ep (VBoolean false) ->
+  BS a (Ctx (InElse ep ei) e) v
+| ECtxInDoWhile : forall a ei ep eo e v,
+  BS a e v ->
+  WhileProducesArg ei ep eo a ->
+  BS a (Ctx (InDoWhile ei ep eo) e) v
+with WhileProducesArg : Expr -> Expr -> Expr -> Value -> Prop :=
+| InitArg : forall a0 ei ep eo vi,
+  BS a0 ei vi ->
+  WhileProducesArg ei ep eo vi
+| NextArg : forall a ei ep eo vi,
+  WhileProducesArg ei ep eo a ->
+  BS a ep (VBoolean true) ->
+  BS a ei vi ->
+  WhileProducesArg ei ep eo vi.
+
+Notation "A ⊢ B ⇓ C" := (BS A B C) (at level 80).
+
+Theorem introduce_if_ctx : forall a ep ei et ee v,
+  a ⊢ If ep ei et ee ⇓ v
+  <->
+  a ⊢ If ep ei (Ctx (InThen ep ei) et) (Ctx (InElse ep ei) ee) ⇓ v
+.
+Proof.
+  split. 
+  - intros.
+    * inversion H.
+      + eapply EIfTrue; auto.
+        -- apply H7.
+        -- eapply ECtxInThen; auto.
+          ** apply H7.
+          ** auto.
+      + eapply EIfFalse; auto.
+        -- apply H7.
+        -- eapply ECtxInElse; auto.
+          ** apply H7.
+          ** auto.
+  - intros. inversion H.
+    * eapply EIfTrue; auto.
+      + apply H7.
+      + inversion H8; auto.
+    * eapply EIfFalse; auto.
+      + apply H7.
+      + inversion H8; auto.
+Qed.
+
+Theorem ctx_allows_const_union : forall a ep n v,
+  a ⊢ Ctx (InThen ep (Num n)) Arg ⇓ v
+  <->
+  a ⊢ Ctx (InThen ep (Num n)) (Num n) ⇓ v
+.
+Proof.
+  split.
+  - intros. inversion H.
+    * eapply ECtxInThen.
+      + subst. inversion H6. subst. inversion H4. subst. apply ENum.
+      + subst. inversion H6. subst. inversion H4. subst. apply ENum.
+      + subst. apply H7.
+  - intros. inversion H.
+    * eapply ECtxInThen.
+      + subst. inversion H6. subst. inversion H4. subst. apply EArg.
+      + subst. inversion H6. subst. inversion H4. subst. apply ENum.
+      + subst. apply H7. 
+Qed.
+
+Theorem ctx_push_down_add : forall assum a l r v,
+  a ⊢ Ctx assum (Add l r) ⇓ v
+  <->
+  a ⊢ Add (Ctx assum l) (Ctx assum r) ⇓ v
+.
+Proof.
+  split.
+  - intros. inversion H; subst.
+    * inversion H2. subst. eapply EAdd; eapply ECtxInThen; eauto.
+    * inversion H2. subst. eapply EAdd; eapply ECtxInElse; eauto.
+    * inversion H3. subst. eapply EAdd; eapply ECtxInDoWhile; eauto.
+  - intros. inversion H. subst. inversion H3; inversion H5; subst.
+    * eapply ECtxInThen; eauto. eapply EAdd; eauto.
+    * inversion H9.
+    * inversion H9.
+    * inversion H9.
+    * eapply ECtxInElse; eauto. eapply EAdd; eauto.
+    * inversion H9.
+    * inversion H8.
+    * inversion H8.
+    * eapply ECtxInDoWhile; eauto. eapply EAdd; eauto.
+Qed.
+
+Theorem ctx_push_down_if : forall assum a ep ei et ee v,
+  a ⊢ Ctx assum (If ep ei et ee) ⇓ v
+  <->
+  a ⊢ If (Ctx assum ep) (Ctx assum ei) et ee ⇓ v
+.
+Proof.
+  split.
+  - intros. inversion H; subst.
+    * inversion H2. subst.
+      + eapply EIfTrue; eauto; eapply ECtxInThen; eauto.
+      + eapply EIfFalse; eauto; eapply ECtxInThen; eauto.
+    * inversion H2. subst.
+      + eapply EIfTrue; eauto; eapply ECtxInElse; eauto.
+      + eapply EIfFalse; eauto; eapply ECtxInElse; eauto.
+    * inversion H3. subst.
+      + eapply EIfTrue; eauto; eapply ECtxInDoWhile; eauto.
+      + eapply EIfFalse; eauto; eapply ECtxInDoWhile; eauto.
+  - intros. inversion H; subst; inversion H6; inversion H7; subst.
+    * eapply ECtxInThen; eauto. eapply EIfTrue; eauto.
+    * inversion H10.
+    * inversion H10.
+    * inversion H10.
+    * eapply ECtxInElse; eauto. eapply EIfTrue; eauto.
+    * inversion H10.
+    * inversion H9.
+    * inversion H9.
+    * eapply ECtxInDoWhile; eauto. eapply EIfTrue; eauto.
+    * eapply ECtxInThen; eauto. eapply EIfFalse; eauto.
+    * inversion H10.
+    * inversion H10.
+    * inversion H10.
+    * eapply ECtxInElse; eauto. eapply EIfFalse; eauto.
+    * inversion H10.
+    * inversion H9.
+    * inversion H9.
+    * eapply ECtxInDoWhile; eauto. eapply EIfFalse; eauto.
+Qed.