Theory SOS

theory SOS
imports Nominal
(*                                                        *)
(* Formalisation of some typical SOS-proofs.              *)
(*                                                        *) 
(* This work was inspired by challenge suggested by Adam  *)
(* Chlipala on the POPLmark mailing list.                 *)
(*                                                        *) 
(* We thank Nick Benton for helping us with the           *) 
(* termination-proof for evaluation.                      *)
(*                                                        *)
(* The formalisation was done by Julien Narboux and       *)
(* Christian Urban.                                       *)

theory SOS
  imports "HOL-Nominal.Nominal"

atom_decl name

text ‹types and terms›
nominal_datatype ty = 
    TVar "nat"
  | Arrow "ty" "ty" ("_→_" [100,100] 100)

nominal_datatype trm = 
    Var "name"
  | Lam "«name»trm" ("Lam [_]._" [100,100] 100)
  | App "trm" "trm"

lemma fresh_ty:
  fixes x::"name" 
  and   T::"ty"
  shows "x♯T"
by (induct T rule: ty.induct)
   (auto simp add: fresh_nat)

text ‹Parallel and single substitution.›
  lookup :: "(name×trm) list ⇒ name ⇒ trm"   
  "lookup [] x        = Var x"
| "lookup ((y,e)#θ) x = (if x=y then e else lookup θ x)"

lemma lookup_eqvt[eqvt]:
  fixes pi::"name prm"
  shows "pi∙(lookup θ X) = lookup (pi∙θ) (pi∙X)"
by (induct θ) (auto simp add: eqvts)

lemma lookup_fresh:
  fixes z::"name"
  assumes a: "z♯θ" and b: "z♯x"
  shows "z ♯lookup θ x"
using a b
by (induct rule: lookup.induct) (auto simp add: fresh_list_cons)

lemma lookup_fresh':
  assumes "z♯θ"
  shows "lookup θ z = Var z"
using assms 
by (induct rule: lookup.induct)
   (auto simp add: fresh_list_cons fresh_prod fresh_atm)

(* parallel substitution *)

  psubst :: "(name×trm) list ⇒ trm ⇒ trm"  ("_<_>" [95,95] 105)
  "θ<(Var x)> = (lookup θ x)"
| "θ<(App e1 e2)> = App (θ<e1>) (θ<e2>)"
| "x♯θ ⟹ θ<(Lam [x].e)> = Lam [x].(θ<e>)"
apply(rule TrueI)+
apply(simp add: abs_fresh)+

lemma psubst_eqvt[eqvt]:
  fixes pi::"name prm" 
  and   t::"trm"
  shows "pi∙(θ<t>) = (pi∙θ)<(pi∙t)>"
by (nominal_induct t avoiding: θ rule: trm.strong_induct)
   (perm_simp add: fresh_bij lookup_eqvt)+

lemma fresh_psubst: 
  fixes z::"name"
  and   t::"trm"
  assumes "z♯t" and "z♯θ"
  shows "z♯(θ<t>)"
using assms
by (nominal_induct t avoiding: z θ t rule: trm.strong_induct)
   (auto simp add: abs_fresh lookup_fresh)

lemma psubst_empty[simp]:
  shows "[]<t> = t"
  by (nominal_induct t rule: trm.strong_induct) 
     (auto simp add: fresh_list_nil)

text ‹Single substitution›
  subst :: "trm ⇒ name ⇒ trm ⇒ trm" ("_[_::=_]" [100,100,100] 100)
  "t[x::=t']  ≡ ([(x,t')])<t>" 

lemma subst[simp]:
  shows "(Var x)[y::=t'] = (if x=y then t' else (Var x))"
  and   "(App t1 t2)[y::=t'] = App (t1[y::=t']) (t2[y::=t'])"
  and   "x♯(y,t') ⟹ (Lam [x].t)[y::=t'] = Lam [x].(t[y::=t'])"
by (simp_all add: fresh_list_cons fresh_list_nil)

lemma fresh_subst:
  fixes z::"name"
  shows "⟦z♯s; (z=y ∨ z♯t)⟧ ⟹ z♯t[y::=s]"
by (nominal_induct t avoiding: z y s rule: trm.strong_induct)
   (auto simp add: abs_fresh fresh_prod fresh_atm)

lemma forget: 
  assumes a: "x♯e" 
  shows "e[x::=e'] = e"
using a
by (nominal_induct e avoiding: x e' rule: trm.strong_induct)
   (auto simp add: fresh_atm abs_fresh)

lemma psubst_subst_psubst:
  assumes h: "x♯θ"
  shows "θ<e>[x::=e'] = ((x,e')#θ)<e>"
  using h
by (nominal_induct e avoiding: θ x e' rule: trm.strong_induct)
   (auto simp add: fresh_list_cons fresh_atm forget lookup_fresh lookup_fresh')

text ‹Typing Judgements›

  valid :: "(name×ty) list ⇒ bool"
  v_nil[intro]:  "valid []"
| v_cons[intro]: "⟦valid Γ;x♯Γ⟧ ⟹ valid ((x,T)#Γ)"

equivariance valid 

  valid_elim[elim]: "valid ((x,T)#Γ)"

lemma valid_insert:
  assumes a: "valid (Δ@[(x,T)]@Γ)"
  shows "valid (Δ @ Γ)" 
using a
by (induct Δ)
   (auto simp add:  fresh_list_append fresh_list_cons elim!: valid_elim)

lemma fresh_set: 
  shows "y♯xs = (∀x∈set xs. y♯x)"
by (induct xs) (simp_all add: fresh_list_nil fresh_list_cons)

lemma context_unique:
  assumes a1: "valid Γ"
  and     a2: "(x,T) ∈ set Γ"
  and     a3: "(x,U) ∈ set Γ"
  shows "T = U" 
using a1 a2 a3
by (induct) (auto simp add: fresh_set fresh_prod fresh_atm)

text ‹Typing Relation›

  typing :: "(name×ty) list⇒trm⇒ty⇒bool" ("_ ⊢ _ : _" [60,60,60] 60) 
  t_Var[intro]:   "⟦valid Γ; (x,T)∈set Γ⟧ ⟹ Γ ⊢ Var x : T"
| t_App[intro]:   "⟦Γ ⊢ e1 : T1→T2; Γ ⊢ e2 : T1⟧ ⟹ Γ ⊢ App e1 e2 : T2"
| t_Lam[intro]:   "⟦x♯Γ; (x,T1)#Γ ⊢ e : T2⟧ ⟹ Γ ⊢ Lam [x].e : T1→T2"

equivariance typing

nominal_inductive typing
  by (simp_all add: abs_fresh fresh_ty)

lemma typing_implies_valid:
  assumes a: "Γ ⊢ t : T"
  shows "valid Γ"
using a by (induct) (auto)

lemma t_App_elim:
  assumes a: "Γ ⊢ App t1 t2 : T"
  obtains T' where "Γ ⊢ t1 : T' → T" and "Γ ⊢ t2 : T'"
using a
by (cases) (auto simp add: trm.inject)

lemma t_Lam_elim: 
  assumes a: "Γ ⊢ Lam [x].t : T" "x♯Γ"
  obtains T1 and T2 where "(x,T1)#Γ ⊢ t : T2" and "T=T1→T2"
using a
by (cases rule: typing.strong_cases [where x="x"])
   (auto simp add: abs_fresh fresh_ty alpha trm.inject)

  "sub_context" :: "(name×ty) list ⇒ (name×ty) list ⇒ bool" ("_ ⊆ _" [55,55] 55)
  1 ⊆ Γ2 ≡ ∀x T. (x,T)∈set Γ1 ⟶ (x,T)∈set Γ2"

lemma weakening: 
  fixes Γ1 Γ2::"(name×ty) list"
  assumes 1 ⊢ e: T" and "valid Γ2" and 1 ⊆ Γ2"
  shows 2 ⊢ e: T"
  using assms
proof(nominal_induct Γ1 e T avoiding: Γ2 rule: typing.strong_induct)
  case (t_Lam x Γ1 T1 t T2 Γ2)
  have vc: "x♯Γ2" by fact
  have ih: "⟦valid ((x,T1)#Γ2); (x,T1)#Γ1 ⊆ (x,T1)#Γ2⟧ ⟹ (x,T1)#Γ2 ⊢ t : T2" by fact
  have "valid Γ2" by fact
  then have "valid ((x,T1)#Γ2)" using vc by auto
  have 1 ⊆ Γ2" by fact
  then have "(x,T1)#Γ1 ⊆ (x,T1)#Γ2" by simp
  ultimately have "(x,T1)#Γ2 ⊢ t : T2" using ih by simp 
  with vc show 2 ⊢ Lam [x].t : T1→T2" by auto
qed (auto)

lemma type_substitutivity_aux:
  assumes a: "(Δ@[(x,T')]@Γ) ⊢ e : T"
  and     b: "Γ ⊢ e' : T'"
  shows "(Δ@Γ) ⊢ e[x::=e'] : T" 
using a b 
proof (nominal_induct Γ"Δ@[(x,T')]@Γ" e T avoiding: e' Δ rule: typing.strong_induct)
  case (t_Var y T e' Δ)
  then have a1: "valid (Δ@[(x,T')]@Γ)" 
       and  a2: "(y,T) ∈ set (Δ@[(x,T')]@Γ)" 
       and  a3: "Γ ⊢ e' : T'" .
  from a1 have a4: "valid (Δ@Γ)" by (rule valid_insert)
  { assume eq: "x=y"
    from a1 a2 have "T=T'" using eq by (auto intro: context_unique)
    with a3 have "Δ@Γ ⊢ Var y[x::=e'] : T" using eq a4 by (auto intro: weakening)
  { assume ineq: "x≠y"
    from a2 have "(y,T) ∈ set (Δ@Γ)" using ineq by simp
    then have "Δ@Γ ⊢ Var y[x::=e'] : T" using ineq a4 by auto
  ultimately show "Δ@Γ ⊢ Var y[x::=e'] : T" by blast
qed (force simp add: fresh_list_append fresh_list_cons)+

corollary type_substitutivity:
  assumes a: "(x,T')#Γ ⊢ e : T"
  and     b: "Γ ⊢ e' : T'"
  shows "Γ ⊢ e[x::=e'] : T"
using a b type_substitutivity_aux[where Δ="[]"]
by (auto)

text ‹Values›
  val :: "trm⇒bool" 
  v_Lam[intro]:   "val (Lam [x].e)"

equivariance val 

lemma not_val_App[simp]:
  "¬ val (App e1 e2)" 
  "¬ val (Var x)"
  by (auto elim: val.cases)

text ‹Big-Step Evaluation›

  big :: "trm⇒trm⇒bool" ("_ ⇓ _" [80,80] 80) 
  b_Lam[intro]:   "Lam [x].e ⇓ Lam [x].e"
| b_App[intro]:   "⟦x♯(e1,e2,e'); e1⇓Lam [x].e; e2⇓e2'; e[x::=e2']⇓e'⟧ ⟹ App e1 e2 ⇓ e'"

equivariance big

nominal_inductive big
  by (simp_all add: abs_fresh)

lemma big_preserves_fresh:
  fixes x::"name"
  assumes a: "e ⇓ e'" "x♯e" 
  shows "x♯e'"
  using a by (induct) (auto simp add: abs_fresh fresh_subst)

lemma b_App_elim:
  assumes a: "App e1 e2 ⇓ e'" "x♯(e1,e2,e')"
  obtains f1 and f2 where "e1 ⇓ Lam [x]. f1" "e2 ⇓ f2" "f1[x::=f2] ⇓ e'"
  using a
by (cases rule: big.strong_cases[where x="x" and xa="x"])
   (auto simp add: trm.inject)

lemma subject_reduction:
  assumes a: "e ⇓ e'" and b: "Γ ⊢ e : T"
  shows "Γ ⊢ e' : T"
  using a b
proof (nominal_induct avoiding: Γ arbitrary: T rule: big.strong_induct) 
  case (b_App x e1 e2 e' e e2' Γ T)
  have vc: "x♯Γ" by fact
  have "Γ ⊢ App e1 e2 : T" by fact
  then obtain T' where a1: "Γ ⊢ e1 : T'→T" and a2: "Γ ⊢ e2 : T'" 
    by (cases) (auto simp add: trm.inject)
  have ih1: "Γ ⊢ e1 : T' → T ⟹ Γ ⊢ Lam [x].e : T' → T" by fact
  have ih2: "Γ ⊢ e2 : T' ⟹ Γ ⊢ e2' : T'" by fact 
  have ih3: "Γ ⊢ e[x::=e2'] : T ⟹ Γ ⊢ e' : T" by fact
  have "Γ ⊢ Lam [x].e : T'→T" using ih1 a1 by simp 
  then have "((x,T')#Γ) ⊢ e : T" using vc  
    by (auto elim: t_Lam_elim simp add: ty.inject)
  have "Γ ⊢ e2': T'" using ih2 a2 by simp
  ultimately have "Γ ⊢ e[x::=e2'] : T" by (simp add: type_substitutivity)
  thus "Γ ⊢ e' : T" using ih3 by simp
qed (blast)

lemma subject_reduction2:
  assumes a: "e ⇓ e'" and b: "Γ ⊢ e : T"
  shows "Γ ⊢ e' : T"
  using a b
by (nominal_induct avoiding: Γ T rule: big.strong_induct)
   (force elim: t_App_elim t_Lam_elim simp add: ty.inject type_substitutivity)+

lemma unicity_of_evaluation:
  assumes a: "e ⇓ e1" 
  and     b: "e ⇓ e2"
  shows "e1 = e2"
  using a b
proof (nominal_induct e e1 avoiding: e2 rule: big.strong_induct)
  case (b_Lam x e t2)
  have "Lam [x].e ⇓ t2" by fact
  thus "Lam [x].e = t2" by cases (simp_all add: trm.inject)
  case (b_App x e1 e2 e' e1' e2' t2)
  have ih1: "⋀t. e1 ⇓ t ⟹ Lam [x].e1' = t" by fact
  have ih2:"⋀t. e2 ⇓ t ⟹ e2' = t" by fact
  have ih3: "⋀t. e1'[x::=e2'] ⇓ t ⟹ e' = t" by fact
  have app: "App e1 e2 ⇓ t2" by fact
  have vc: "x♯e1" "x♯e2" "x♯t2" by fact+
  then have "x♯App e1 e2" by auto
  from app vc obtain f1 f2 where x1: "e1 ⇓ Lam [x]. f1" and x2: "e2 ⇓ f2" and x3: "f1[x::=f2] ⇓ t2"
    by (auto elim!: b_App_elim)
  then have "Lam [x]. f1 = Lam [x]. e1'" using ih1 by simp
  have "f1 = e1'" by (auto simp add: trm.inject alpha) 
  have "f2 = e2'" using x2 ih2 by simp
  ultimately have "e1'[x::=e2'] ⇓ t2" using x3 by simp
  thus "e' = t2" using ih3 by simp

lemma reduces_evaluates_to_values:
  assumes h: "t ⇓ t'"
  shows "val t'"
using h by (induct) (auto)

(* Valuation *)

  V :: "ty ⇒ trm set" 
  "V (TVar x) = {e. val e}"
| "V (T1 → T2) = {Lam [x].e | x e. ∀ v ∈ (V T1). ∃ v'. e[x::=v] ⇓ v' ∧ v' ∈ V T2}"
  by (rule TrueI)+ 

lemma V_eqvt:
  fixes pi::"name prm"
  assumes a: "x∈V T"
  shows "(pi∙x)∈V T"
using a
apply(nominal_induct T arbitrary: pi x rule: ty.strong_induct)
apply(auto simp add: trm.inject)
apply(simp add: eqvts)
apply(rule_tac x="pi∙xa" in exI)
apply(rule_tac x="pi∙e" in exI)
apply(drule_tac x="(rev pi)∙v" in bspec)
apply(rule_tac x="pi∙v'" in exI)
apply(drule_tac pi="pi" in big.eqvt)
apply(perm_simp add: eqvts)

lemma V_arrow_elim_weak:
  assumes h:"u ∈ V (T1 → T2)"
  obtains a t where "u = Lam [a].t" and "∀ v ∈ (V T1). ∃ v'. t[a::=v] ⇓ v' ∧ v' ∈ V T2"
using h by (auto)

lemma V_arrow_elim_strong:
  fixes c::"'a::fs_name"
  assumes h: "u ∈ V (T1 → T2)"
  obtains a t where "a♯c" "u = Lam [a].t" "∀v ∈ (V T1). ∃ v'. t[a::=v] ⇓ v' ∧ v' ∈ V T2"
using h
apply -
apply(erule V_arrow_elim_weak)
apply(subgoal_tac "∃a'::name. a'♯(a,t,c)") (*A*)
apply(erule exE)
apply(drule_tac x="a'" in meta_spec)
apply(drule_tac x="[(a,a')]∙t" in meta_spec)
apply(drule meta_mp)
apply(drule meta_mp)
apply(simp add: trm.inject alpha fresh_left fresh_prod calc_atm fresh_atm)
apply(drule meta_mp)
apply(rule ballI)
apply(drule_tac x="[(a,a')]∙v" in bspec)
apply(simp add: V_eqvt)
apply(rule_tac x="[(a,a')]∙v'" in exI)
apply(drule_tac pi="[(a,a')]" in big.eqvt)
apply(perm_simp add: eqvts calc_atm)
apply(simp add: V_eqvt)
apply(rule exists_fresh')
apply(simp add: fin_supp)

lemma Vs_are_values:
  assumes a: "e ∈ V T"
  shows "val e"
using a by (nominal_induct T arbitrary: e rule: ty.strong_induct) (auto)

lemma values_reduce_to_themselves:
  assumes a: "val v"
  shows "v ⇓ v"
using a by (induct) (auto)

lemma Vs_reduce_to_themselves:
  assumes a: "v ∈ V T"
  shows "v ⇓ v"
using a by (simp add: values_reduce_to_themselves Vs_are_values)

text ‹'θ maps x to e' asserts that θ substitutes x with e›
  mapsto :: "(name×trm) list ⇒ name ⇒ trm ⇒ bool" ("_ maps _ to _" [55,55,55] 55) 
 "θ maps x to e ≡ (lookup θ x) = e"

  v_closes :: "(name×trm) list ⇒ (name×ty) list ⇒ bool" ("_ Vcloses _" [55,55] 55) 
  "θ Vcloses Γ ≡ ∀x T. (x,T) ∈ set Γ ⟶ (∃v. θ maps x to v ∧ v ∈ V T)"

lemma case_distinction_on_context:
  fixes Γ::"(name×ty) list"
  assumes asm1: "valid ((m,t)#Γ)" 
  and     asm2: "(n,U) ∈ set ((m,T)#Γ)"
  shows "(n,U) = (m,T) ∨ ((n,U) ∈ set Γ ∧ n ≠ m)"
proof -
  from asm2 have "(n,U) ∈ set [(m,T)] ∨ (n,U) ∈ set Γ" by auto
  { assume eq: "m=n"
    assume "(n,U) ∈ set Γ" 
    then have "¬ n♯Γ" 
      by (induct Γ) (auto simp add: fresh_list_cons fresh_prod fresh_atm)
    moreover have "m♯Γ" using asm1 by auto
    ultimately have False using eq by auto
  ultimately show ?thesis by auto

lemma monotonicity:
  fixes m::"name"
  fixes θ::"(name × trm) list" 
  assumes h1: "θ Vcloses Γ"
  and     h2: "e ∈ V T" 
  and     h3: "valid ((x,T)#Γ)"
  shows "(x,e)#θ Vcloses (x,T)#Γ"
proof(intro strip)
  fix x' T'
  assume "(x',T') ∈ set ((x,T)#Γ)"
  then have "((x',T')=(x,T)) ∨ ((x',T')∈set Γ ∧ x'≠x)" using h3 
    by (rule_tac case_distinction_on_context)
  { (* first case *)
    assume "(x',T') = (x,T)"
    then have "∃e'. ((x,e)#θ) maps x to e' ∧ e' ∈ V T'" using h2 by auto
  { (* second case *)
    assume "(x',T') ∈ set Γ" and neq:"x' ≠ x"
      then have "∃e'. θ maps x' to e' ∧ e' ∈ V T'" using h1 by auto
      then have "∃e'. ((x,e)#θ) maps x' to e' ∧ e' ∈ V T'" using neq by auto
  ultimately show "∃e'.  ((x,e)#θ) maps x' to e'  ∧ e' ∈ V T'" by blast

lemma termination_aux:
  assumes h1: "Γ ⊢ e : T"
  and     h2: "θ Vcloses Γ"
  shows "∃v. θ<e> ⇓ v ∧ v ∈ V T" 
using h2 h1
proof(nominal_induct e avoiding: Γ θ arbitrary: T rule: trm.strong_induct)
  case (App e1 e2 Γ θ T)
  have ih1: "⋀θ Γ T. ⟦θ Vcloses Γ; Γ ⊢ e1 : T⟧ ⟹ ∃v. θ<e1> ⇓ v ∧ v ∈ V T" by fact
  have ih2: "⋀θ Γ T. ⟦θ Vcloses Γ; Γ ⊢ e2 : T⟧ ⟹ ∃v. θ<e2> ⇓ v ∧ v ∈ V T" by fact
  have as1: "θ Vcloses Γ" by fact 
  have as2: "Γ ⊢ App e1 e2 : T" by fact
  then obtain T' where "Γ ⊢ e1 : T' → T" and "Γ ⊢ e2 : T'" by (auto elim: t_App_elim)
  then obtain v1 v2 where "(i)": "θ<e1> ⇓ v1" "v1 ∈ V (T' → T)"
                      and "(ii)": "θ<e2> ⇓ v2" "v2 ∈ V T'" using ih1 ih2 as1 by blast
  from "(i)" obtain x e' 
            where "v1 = Lam [x].e'" 
            and "(iii)": "(∀v ∈ (V T').∃ v'. e'[x::=v] ⇓ v' ∧ v' ∈ V T)"
            and "(iv)":  "θ<e1> ⇓ Lam [x].e'" 
            and fr: "x♯(θ,e1,e2)" by (blast elim: V_arrow_elim_strong)
  from fr have fr1: "x♯θ<e1>" and fr2: "x♯θ<e2>" by (simp_all add: fresh_psubst)
  from "(ii)" "(iii)" obtain v3 where "(v)": "e'[x::=v2] ⇓ v3" "v3 ∈ V T" by auto
  from fr2 "(ii)" have "x♯v2" by (simp add: big_preserves_fresh)
  then have "x♯e'[x::=v2]" by (simp add: fresh_subst)
  then have fr3: "x♯v3" using "(v)" by (simp add: big_preserves_fresh)
  from fr1 fr2 fr3 have "x♯(θ<e1>,θ<e2>,v3)" by simp
  with "(iv)" "(ii)" "(v)" have "App (θ<e1>) (θ<e2>) ⇓ v3" by auto
  then show "∃v. θ<App e1 e2> ⇓ v ∧ v ∈ V T" using "(v)" by auto
  case (Lam x e Γ θ T)
  have ih:"⋀θ Γ T. ⟦θ Vcloses Γ; Γ ⊢ e : T⟧ ⟹ ∃v. θ<e> ⇓ v ∧ v ∈ V T" by fact
  have as1: "θ Vcloses Γ" by fact
  have as2: "Γ ⊢ Lam [x].e : T" by fact
  have fs: "x♯Γ" "x♯θ" by fact+
  from as2 fs obtain T1 T2 
    where "(i)": "(x,T1)#Γ ⊢ e:T2" and "(ii)": "T = T1 → T2" using fs
    by (auto elim: t_Lam_elim)
  from "(i)" have "(iii)": "valid ((x,T1)#Γ)" by (simp add: typing_implies_valid)
  have "∀v ∈ (V T1). ∃v'. (θ<e>)[x::=v] ⇓ v' ∧ v' ∈ V T2"
    fix v
    assume "v ∈ (V T1)"
    with "(iii)" as1 have "(x,v)#θ Vcloses (x,T1)#Γ" using monotonicity by auto
    with ih "(i)" obtain v' where "((x,v)#θ)<e> ⇓ v' ∧ v' ∈ V T2" by blast
    then have "θ<e>[x::=v] ⇓ v' ∧ v' ∈ V T2" using fs by (simp add: psubst_subst_psubst)
    then show "∃v'. θ<e>[x::=v] ⇓ v' ∧ v' ∈ V T2" by auto
  then have "Lam[x].θ<e> ∈ V (T1 → T2)" by auto
  then have "θ<Lam [x].e> ⇓ Lam [x].θ<e> ∧ Lam [x].θ<e> ∈ V (T1→T2)" using fs by auto
  thus "∃v. θ<Lam [x].e> ⇓ v ∧ v ∈ V T" using "(ii)" by auto
  case (Var x Γ θ T)
  have "Γ ⊢ (Var x) : T" by fact
  then have "(x,T)∈set Γ" by (cases) (auto simp add: trm.inject)
  with Var have "θ<Var x> ⇓ θ<Var x> ∧ θ<Var x>∈ V T" 
    by (auto intro!: Vs_reduce_to_themselves)
  then show "∃v. θ<Var x> ⇓ v ∧ v ∈ V T" by auto

theorem termination_of_evaluation:
  assumes a: "[] ⊢ e : T"
  shows "∃v. e ⇓ v ∧ val v"
proof -
  from a have "∃v. []<e> ⇓ v ∧ v ∈ V T" by (rule termination_aux) (auto)
  thus "∃v. e ⇓ v ∧ val v" using Vs_are_values by auto