Theory Standardization

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theory Standardization
imports Nominal

(*  Title:      HOL/Nominal/Examples/Standardization.thy
    Author:     Stefan Berghofer and Tobias Nipkow
    Copyright   2005, 2008 TU Muenchen
*)

header {* Standardization *}

theory Standardization
imports Nominal
begin

text {*
The proof of the standardization theorem, as well as most of the theorems about
lambda calculus in the following sections, are taken from @{text "HOL/Lambda"}.
*}

subsection {* Lambda terms *}

atom_decl name

nominal_datatype lam =
  Var "name"
| App "lam" "lam" (infixl "°" 200)
| Lam "«name»lam" ("Lam [_]._" [0, 10] 10)

instantiation lam :: size
begin

nominal_primrec size_lam
where
  "size (Var n) = 0"
| "size (t ° u) = size t + size u + 1"
| "size (Lam [x].t) = size t + 1"
  apply finite_guess+
  apply (rule TrueI)+
  apply (simp add: fresh_nat)
  apply fresh_guess+
  done

instance ..

end

nominal_primrec
  subst :: "lam => name => lam => lam"  ("_[_::=_]" [300, 0, 0] 300)
where
  subst_Var: "(Var x)[y::=s] = (if x=y then s else (Var x))"
| subst_App: "(t1 ° t2)[y::=s] = t1[y::=s] ° t2[y::=s]"
| subst_Lam: "x \<sharp> (y, s) ==> (Lam [x].t)[y::=s] = (Lam [x].(t[y::=s]))"
  apply(finite_guess)+
  apply(rule TrueI)+
  apply(simp add: abs_fresh)
  apply(fresh_guess)+
  done

lemma subst_eqvt [eqvt]:
  "(pi::name prm) • (t[x::=u]) = (pi • t)[(pi • x)::=(pi • u)]"
  by (nominal_induct t avoiding: x u rule: lam.strong_induct)
    (perm_simp add: fresh_bij)+

lemma subst_rename:
  "y \<sharp> t ==> ([(y, x)] • t)[y::=u] = t[x::=u]"
  by (nominal_induct t avoiding: x y u rule: lam.strong_induct)
    (simp_all add: fresh_atm calc_atm abs_fresh)

lemma fresh_subst: 
  "(x::name) \<sharp> t ==> x \<sharp> u ==> x \<sharp> t[y::=u]"
  by (nominal_induct t avoiding: x y u rule: lam.strong_induct)
    (auto simp add: abs_fresh fresh_atm)

lemma fresh_subst': 
  "(x::name) \<sharp> u ==> x \<sharp> t[x::=u]"
  by (nominal_induct t avoiding: x u rule: lam.strong_induct)
    (auto simp add: abs_fresh fresh_atm)

lemma subst_forget: "(x::name) \<sharp> t ==> t[x::=u] = t"
  by (nominal_induct t avoiding: x u rule: lam.strong_induct)
    (auto simp add: abs_fresh fresh_atm)

lemma subst_subst:
  "x ≠ y ==> x \<sharp> v ==> t[y::=v][x::=u[y::=v]] = t[x::=u][y::=v]"
  by (nominal_induct t avoiding: x y u v rule: lam.strong_induct)
    (auto simp add: fresh_subst subst_forget)

declare subst_Var [simp del]

lemma subst_eq [simp]: "(Var x)[x::=u] = u"
  by (simp add: subst_Var)

lemma subst_neq [simp]: "x ≠ y ==> (Var x)[y::=u] = Var x"
  by (simp add: subst_Var)

inductive beta :: "lam => lam => bool"  (infixl "->β" 50)
  where
    beta: "x \<sharp> t ==> (Lam [x].s) ° t ->β s[x::=t]"
  | appL [simp, intro!]: "s ->β t ==> s ° u ->β t ° u"
  | appR [simp, intro!]: "s ->β t ==> u ° s ->β u ° t"
  | abs [simp, intro!]: "s ->β t ==> (Lam [x].s) ->β (Lam [x].t)"

equivariance beta
nominal_inductive beta
  by (simp_all add: abs_fresh fresh_subst')

lemma better_beta [simp, intro!]: "(Lam [x].s) ° t ->β s[x::=t]"
proof -
  obtain y::name where y: "y \<sharp> (x, s, t)"
    by (rule exists_fresh) (rule fin_supp)
  then have "y \<sharp> t" by simp
  then have "(Lam [y]. [(y, x)] • s) ° t ->β ([(y, x)] • s)[y::=t]"
    by (rule beta)
  moreover from y have "(Lam [x].s) = (Lam [y]. [(y, x)] • s)"
    by (auto simp add: lam.inject alpha' fresh_prod fresh_atm)
  ultimately show ?thesis using y by (simp add: subst_rename)
qed

abbreviation
  beta_reds :: "lam => lam => bool"  (infixl "->β*" 50) where
  "s ->β* t ≡ beta** s t"


subsection {* Application of a term to a list of terms *}

abbreviation
  list_application :: "lam => lam list => lam"  (infixl "°°" 150) where
  "t °° ts ≡ foldl (op °) t ts"

lemma apps_eq_tail_conv [iff]: "(r °° ts = s °° ts) = (r = s)"
  by (induct ts rule: rev_induct) (auto simp add: lam.inject)

lemma Var_eq_apps_conv [iff]: "(Var m = s °° ss) = (Var m = s ∧ ss = [])"
  by (induct ss arbitrary: s) auto

lemma Var_apps_eq_Var_apps_conv [iff]:
    "(Var m °° rs = Var n °° ss) = (m = n ∧ rs = ss)"
  apply (induct rs arbitrary: ss rule: rev_induct)
   apply (simp add: lam.inject)
   apply blast
  apply (induct_tac ss rule: rev_induct)
   apply (auto simp add: lam.inject)
  done

lemma App_eq_foldl_conv:
  "(r ° s = t °° ts) =
    (if ts = [] then r ° s = t
    else (∃ss. ts = ss @ [s] ∧ r = t °° ss))"
  apply (rule_tac xs = ts in rev_exhaust)
   apply (auto simp add: lam.inject)
  done

lemma Abs_eq_apps_conv [iff]:
    "((Lam [x].r) = s °° ss) = ((Lam [x].r) = s ∧ ss = [])"
  by (induct ss rule: rev_induct) auto

lemma apps_eq_Abs_conv [iff]: "(s °° ss = (Lam [x].r)) = (s = (Lam [x].r) ∧ ss = [])"
  by (induct ss rule: rev_induct) auto

lemma Abs_App_neq_Var_apps [iff]:
    "(Lam [x].s) ° t ≠ Var n °° ss"
  by (induct ss arbitrary: s t rule: rev_induct) (auto simp add: lam.inject)

lemma Var_apps_neq_Abs_apps [iff]:
    "Var n °° ts ≠ (Lam [x].r) °° ss"
  apply (induct ss arbitrary: ts rule: rev_induct)
   apply simp
  apply (induct_tac ts rule: rev_induct)
   apply (auto simp add: lam.inject)
  done

lemma ex_head_tail:
  "∃ts h. t = h °° ts ∧ ((∃n. h = Var n) ∨ (∃x u. h = (Lam [x].u)))"
  apply (induct t rule: lam.induct)
    apply (rule_tac x = "[]" in exI)
    apply (simp add: lam.inject)
   apply clarify
   apply (rename_tac ts1 ts2 h1 h2)
   apply (rule_tac x = "ts1 @ [h2 °° ts2]" in exI)
   apply (simp add: lam.inject)
  apply simp
  apply blast
  done

lemma size_apps [simp]:
  "size (r °° rs) = size r + foldl (op +) 0 (map size rs) + length rs"
  by (induct rs rule: rev_induct) auto

lemma lem0: "(0::nat) < k ==> m ≤ n ==> m < n + k"
  by simp

lemma subst_map [simp]:
    "(t °° ts)[x::=u] = t[x::=u] °° map (λt. t[x::=u]) ts"
  by (induct ts arbitrary: t) simp_all

lemma app_last: "(t °° ts) ° u = t °° (ts @ [u])"
  by simp

lemma perm_apps [eqvt]:
  "(pi::name prm) • (t °° ts) = ((pi • t) °° (pi • ts))"
  by (induct ts rule: rev_induct) (auto simp add: append_eqvt)

lemma fresh_apps [simp]: "(x::name) \<sharp> (t °° ts) = (x \<sharp> t ∧ x \<sharp> ts)"
  by (induct ts rule: rev_induct)
    (auto simp add: fresh_list_append fresh_list_nil fresh_list_cons)

text {* A customized induction schema for @{text "°°"}. *}

lemma lem:
  assumes "!!n ts (z::'a::fs_name). (!!z. ∀t ∈ set ts. P z t) ==> P z (Var n °° ts)"
    and "!!x u ts z. x \<sharp> z ==> (!!z. P z u) ==> (!!z. ∀t ∈ set ts. P z t) ==> P z ((Lam [x].u) °° ts)"
  shows "size t = n ==> P z t"
  apply (induct n arbitrary: t z rule: nat_less_induct)
  apply (cut_tac t = t in ex_head_tail)
  apply clarify
  apply (erule disjE)
   apply clarify
   apply (rule assms)
   apply clarify
   apply (erule allE, erule impE)
    prefer 2
    apply (erule allE, erule impE, rule refl, erule spec)
   apply simp
   apply (rule lem0)
    apply force
   apply (rule elem_le_sum)
   apply force
  apply clarify
  apply (subgoal_tac "∃y::name. y \<sharp> (x, u, z)")
  prefer 2
  apply (rule exists_fresh')
  apply (rule fin_supp)
  apply (erule exE)
  apply (subgoal_tac "(Lam [x].u) = (Lam [y].([(y, x)] • u))")
  prefer 2
  apply (auto simp add: lam.inject alpha' fresh_prod fresh_atm)[]
  apply (simp (no_asm_simp))
  apply (rule assms)
  apply (simp add: fresh_prod)
   apply (erule allE, erule impE)
    prefer 2
    apply (erule allE, erule impE, rule refl, erule spec)
   apply simp
  apply clarify
  apply (erule allE, erule impE)
   prefer 2
   apply (erule allE, erule impE, rule refl, erule spec)
  apply simp
  apply (rule le_imp_less_Suc)
  apply (rule trans_le_add1)
  apply (rule trans_le_add2)
  apply (rule elem_le_sum)
  apply force
  done

theorem Apps_lam_induct:
  assumes "!!n ts (z::'a::fs_name). (!!z. ∀t ∈ set ts. P z t) ==> P z (Var n °° ts)"
    and "!!x u ts z. x \<sharp> z ==> (!!z. P z u) ==> (!!z. ∀t ∈ set ts. P z t) ==> P z ((Lam [x].u) °° ts)"
  shows "P z t"
  apply (rule_tac t = t and z = z in lem)
    prefer 3
    apply (rule refl)
    using assms apply blast+
  done


subsection {* Congruence rules *}

lemma apps_preserves_beta [simp]:
    "r ->β s ==> r °° ss ->β s °° ss"
  by (induct ss rule: rev_induct) auto

lemma rtrancl_beta_Abs [intro!]:
    "s ->β* s' ==> (Lam [x].s) ->β* (Lam [x].s')"
  by (induct set: rtranclp) (blast intro: rtranclp.rtrancl_into_rtrancl)+

lemma rtrancl_beta_AppL:
    "s ->β* s' ==> s ° t ->β* s' ° t"
  by (induct set: rtranclp) (blast intro: rtranclp.rtrancl_into_rtrancl)+

lemma rtrancl_beta_AppR:
    "t ->β* t' ==> s ° t ->β* s ° t'"
  by (induct set: rtranclp) (blast intro: rtranclp.rtrancl_into_rtrancl)+

lemma rtrancl_beta_App [intro]:
    "s ->β* s' ==> t ->β* t' ==> s ° t ->β* s' ° t'"
  by (blast intro!: rtrancl_beta_AppL rtrancl_beta_AppR intro: rtranclp_trans)


subsection {* Lifting an order to lists of elements *}

definition
  step1 :: "('a => 'a => bool) => 'a list => 'a list => bool" where
  "step1 r =
    (λys xs. ∃us z z' vs. xs = us @ z # vs ∧ r z' z ∧ ys =
      us @ z' # vs)"

lemma not_Nil_step1 [iff]: "¬ step1 r [] xs"
  apply (unfold step1_def)
  apply blast
  done

lemma not_step1_Nil [iff]: "¬ step1 r xs []"
  apply (unfold step1_def)
  apply blast
  done

lemma Cons_step1_Cons [iff]:
    "(step1 r (y # ys) (x # xs)) =
      (r y x ∧ xs = ys ∨ x = y ∧ step1 r ys xs)"
  apply (unfold step1_def)
  apply (rule iffI)
   apply (erule exE)
   apply (rename_tac ts)
   apply (case_tac ts)
    apply fastsimp
   apply force
  apply (erule disjE)
   apply blast
  apply (blast intro: Cons_eq_appendI)
  done

lemma append_step1I:
  "step1 r ys xs ∧ vs = us ∨ ys = xs ∧ step1 r vs us
    ==> step1 r (ys @ vs) (xs @ us)"
  apply (unfold step1_def)
  apply auto
   apply blast
  apply (blast intro: append_eq_appendI)
  done

lemma Cons_step1E [elim!]:
  assumes "step1 r ys (x # xs)"
    and "!!y. ys = y # xs ==> r y x ==> R"
    and "!!zs. ys = x # zs ==> step1 r zs xs ==> R"
  shows R
  using assms
  apply (cases ys)
   apply (simp add: step1_def)
  apply blast
  done

lemma Snoc_step1_SnocD:
  "step1 r (ys @ [y]) (xs @ [x])
    ==> (step1 r ys xs ∧ y = x ∨ ys = xs ∧ r y x)"
  apply (unfold step1_def)
  apply (clarify del: disjCI)
  apply (rename_tac vs)
  apply (rule_tac xs = vs in rev_exhaust)
   apply force
  apply simp
  apply blast
  done


subsection {* Lifting beta-reduction to lists *}

abbreviation
  list_beta :: "lam list => lam list => bool"  (infixl "[->β]1" 50) where
  "rs [->β]1 ss ≡ step1 beta rs ss"

lemma head_Var_reduction:
  "Var n °° rs ->β v ==> ∃ss. rs [->β]1 ss ∧ v = Var n °° ss"
  apply (induct u  "Var n °° rs" v arbitrary: rs set: beta)
     apply simp
    apply (rule_tac xs = rs in rev_exhaust)
     apply simp
    apply (atomize, force intro: append_step1I iff: lam.inject)
   apply (rule_tac xs = rs in rev_exhaust)
    apply simp
    apply (auto 0 3 intro: disjI2 [THEN append_step1I] simp add: lam.inject)
  done

lemma apps_betasE [case_names appL appR beta, consumes 1]:
  assumes major: "r °° rs ->β s"
    and cases: "!!r'. r ->β r' ==> s = r' °° rs ==> R"
      "!!rs'. rs [->β]1 rs' ==> s = r °° rs' ==> R"
      "!!t u us. (x \<sharp> r ==> r = (Lam [x].t) ∧ rs = u # us ∧ s = t[x::=u] °° us) ==> R"
  shows R
proof -
  from major have
   "(∃r'. r ->β r' ∧ s = r' °° rs) ∨
    (∃rs'. rs [->β]1 rs' ∧ s = r °° rs') ∨
    (∃t u us. x \<sharp> r --> r = (Lam [x].t) ∧ rs = u # us ∧ s = t[x::=u] °° us)"
    apply (nominal_induct u  "r °° rs" s avoiding: x r rs rule: beta.strong_induct)
    apply (simp add: App_eq_foldl_conv)
    apply (split split_if_asm)
    apply simp
    apply blast
    apply simp
    apply (rule impI)+
    apply (rule disjI2)
    apply (rule disjI2)
    apply (subgoal_tac "r = [(xa, x)] • (Lam [x].s)")
    prefer 2
    apply (simp add: perm_fresh_fresh)
    apply (drule conjunct1)
    apply (subgoal_tac "r = (Lam [xa]. [(xa, x)] • s)")
    prefer 2
    apply (simp add: calc_atm)
    apply (thin_tac "r = ?t")
    apply simp
    apply (rule exI)
    apply (rule conjI)
    apply (rule refl)
    apply (simp add: abs_fresh fresh_atm fresh_left calc_atm subst_rename)
      apply (drule App_eq_foldl_conv [THEN iffD1])
      apply (split split_if_asm)
       apply simp
       apply blast
      apply (force intro!: disjI1 [THEN append_step1I] simp add: fresh_list_append)
     apply (drule App_eq_foldl_conv [THEN iffD1])
     apply (split split_if_asm)
      apply simp
      apply blast
     apply (clarify, auto 0 3 intro!: exI intro: append_step1I)
    done
  with cases show ?thesis by blast
qed

lemma apps_preserves_betas [simp]:
    "rs [->β]1 ss ==> r °° rs ->β r °° ss"
  apply (induct rs arbitrary: ss rule: rev_induct)
   apply simp
  apply simp
  apply (rule_tac xs = ss in rev_exhaust)
   apply simp
  apply simp
  apply (drule Snoc_step1_SnocD)
  apply blast
  done


subsection {* Standard reduction relation *}

text {*
Based on lecture notes by Ralph Matthes,
original proof idea due to Ralph Loader.
*}

declare listrel_mono [mono_set]

lemma listrelp_eqvt [eqvt]:
  assumes xy: "listrelp f (x::'a::pt_name list) y"
  shows "listrelp ((pi::name prm) • f) (pi • x) (pi • y)" using xy
  apply induct
  apply simp
  apply (rule listrelp.intros)
  apply simp
  apply (rule listrelp.intros)
  apply (drule_tac pi=pi in perm_boolI)
  apply perm_simp
  apply assumption
  done

inductive
  sred :: "lam => lam => bool"  (infixl "->s" 50)
  and sredlist :: "lam list => lam list => bool"  (infixl "[->s]" 50)
where
  "s [->s] t ≡ listrelp op ->s s t"
| Var: "rs [->s] rs' ==> Var x °° rs ->s Var x °° rs'"
| Abs: "x \<sharp> (ss, ss') ==> r ->s r' ==> ss [->s] ss' ==> (Lam [x].r) °° ss ->s (Lam [x].r') °° ss'"
| Beta: "x \<sharp> (s, ss, t) ==> r[x::=s] °° ss ->s t ==> (Lam [x].r) ° s °° ss ->s t"

equivariance sred
nominal_inductive sred
  by (simp add: abs_fresh)+

lemma better_sred_Abs:
  assumes H1: "r ->s r'"
  and H2: "ss [->s] ss'"
  shows "(Lam [x].r) °° ss ->s (Lam [x].r') °° ss'"
proof -
  obtain y::name where y: "y \<sharp> (x, r, r', ss, ss')"
    by (rule exists_fresh) (rule fin_supp)
  then have "y \<sharp> (ss, ss')" by simp
  moreover from H1 have "[(y, x)] • (r ->s r')" by (rule perm_boolI)
  then have "([(y, x)] • r) ->s ([(y, x)] • r')" by (simp add: eqvts)
  ultimately have "(Lam [y]. [(y, x)] • r) °° ss ->s (Lam [y]. [(y, x)] • r') °° ss'" using H2
    by (rule sred.Abs)
  moreover from y have "(Lam [x].r) = (Lam [y]. [(y, x)] • r)"
    by (auto simp add: lam.inject alpha' fresh_prod fresh_atm)
  moreover from y have "(Lam [x].r') = (Lam [y]. [(y, x)] • r')"
    by (auto simp add: lam.inject alpha' fresh_prod fresh_atm)
  ultimately show ?thesis by simp
qed

lemma better_sred_Beta:
  assumes H: "r[x::=s] °° ss ->s t"
  shows "(Lam [x].r) ° s °° ss ->s t"
proof -
  obtain y::name where y: "y \<sharp> (x, r, s, ss, t)"
    by (rule exists_fresh) (rule fin_supp)
  then have "y \<sharp> (s, ss, t)" by simp
  moreover from y H have "([(y, x)] • r)[y::=s] °° ss ->s t"
    by (simp add: subst_rename)
  ultimately have "(Lam [y].[(y, x)] • r) ° s °° ss ->s t"
    by (rule sred.Beta)
  moreover from y have "(Lam [x].r) = (Lam [y]. [(y, x)] • r)"
    by (auto simp add: lam.inject alpha' fresh_prod fresh_atm)
  ultimately show ?thesis by simp
qed

lemmas better_sred_intros = sred.Var better_sred_Abs better_sred_Beta

lemma refl_listrelp: "∀x∈set xs. R x x ==> listrelp R xs xs"
  by (induct xs) (auto intro: listrelp.intros)

lemma refl_sred: "t ->s t"
  by (nominal_induct t rule: Apps_lam_induct) (auto intro: refl_listrelp better_sred_intros)

lemma listrelp_conj1: "listrelp (λx y. R x y ∧ S x y) x y ==> listrelp R x y"
  by (erule listrelp.induct) (auto intro: listrelp.intros)

lemma listrelp_conj2: "listrelp (λx y. R x y ∧ S x y) x y ==> listrelp S x y"
  by (erule listrelp.induct) (auto intro: listrelp.intros)

lemma listrelp_app:
  assumes xsys: "listrelp R xs ys"
  shows "listrelp R xs' ys' ==> listrelp R (xs @ xs') (ys @ ys')" using xsys
  by (induct arbitrary: xs' ys') (auto intro: listrelp.intros)

lemma lemma1:
  assumes r: "r ->s r'" and s: "s ->s s'"
  shows "r ° s ->s r' ° s'" using r
proof induct
  case (Var rs rs' x)
  then have "rs [->s] rs'" by (rule listrelp_conj1)
  moreover have "[s] [->s] [s']" by (iprover intro: s listrelp.intros)
  ultimately have "rs @ [s] [->s] rs' @ [s']" by (rule listrelp_app)
  hence "Var x °° (rs @ [s]) ->s Var x °° (rs' @ [s'])" by (rule sred.Var)
  thus ?case by (simp only: app_last)
next
  case (Abs x ss ss' r r')
  from Abs(4) have "ss [->s] ss'" by (rule listrelp_conj1)
  moreover have "[s] [->s] [s']" by (iprover intro: s listrelp.intros)
  ultimately have "ss @ [s] [->s] ss' @ [s']" by (rule listrelp_app)
  with `r ->s r'` have "(Lam [x].r) °° (ss @ [s]) ->s (Lam [x].r') °° (ss' @ [s'])"
    by (rule better_sred_Abs)
  thus ?case by (simp only: app_last)
next
  case (Beta x u ss t r)
  hence "r[x::=u] °° (ss @ [s]) ->s t ° s'" by (simp only: app_last)
  hence "(Lam [x].r) ° u °° (ss @ [s]) ->s t ° s'" by (rule better_sred_Beta)
  thus ?case by (simp only: app_last)
qed

lemma lemma1':
  assumes ts: "ts [->s] ts'"
  shows "r ->s r' ==> r °° ts ->s r' °° ts'" using ts
  by (induct arbitrary: r r') (auto intro: lemma1)

lemma listrelp_betas:
  assumes ts: "listrelp op ->β* ts ts'"
  shows "!!t t'. t ->β* t' ==> t °° ts ->β* t' °° ts'" using ts
  by induct auto

lemma lemma2:
  assumes t: "t ->s u"
  shows "t ->β* u" using t
  by induct (auto dest: listrelp_conj2
    intro: listrelp_betas apps_preserves_beta converse_rtranclp_into_rtranclp)

lemma lemma3:
  assumes r: "r ->s r'"
  shows "s ->s s' ==> r[x::=s] ->s r'[x::=s']" using r
proof (nominal_induct avoiding: x s s' rule: sred.strong_induct)
  case (Var rs rs' y)
  hence "map (λt. t[x::=s]) rs [->s] map (λt. t[x::=s']) rs'"
    by induct (auto intro: listrelp.intros Var)
  moreover have "Var y[x::=s] ->s Var y[x::=s']"
    by (cases "y = x") (auto simp add: Var intro: refl_sred)
  ultimately show ?case by simp (rule lemma1')
next
  case (Abs y ss ss' r r')
  then have "r[x::=s] ->s r'[x::=s']" by fast
  moreover from Abs(8) `s ->s s'` have "map (λt. t[x::=s]) ss [->s] map (λt. t[x::=s']) ss'"
    by induct (auto intro: listrelp.intros Abs)
  ultimately show ?case using Abs(6) `y \<sharp> x` `y \<sharp> s` `y \<sharp> s'`
    by simp (rule better_sred_Abs)
next
  case (Beta y u ss t r)
  thus ?case by (auto simp add: subst_subst fresh_atm intro: better_sred_Beta)
qed

lemma lemma4_aux:
  assumes rs: "listrelp (λt u. t ->s u ∧ (∀r. u ->β r --> t ->s r)) rs rs'"
  shows "rs' [->β]1 ss ==> rs [->s] ss" using rs
proof (induct arbitrary: ss)
  case Nil
  thus ?case by cases (auto intro: listrelp.Nil)
next
  case (Cons x y xs ys)
  note Cons' = Cons
  show ?case
  proof (cases ss)
    case Nil with Cons show ?thesis by simp
  next
    case (Cons y' ys')
    hence ss: "ss = y' # ys'" by simp
    from Cons Cons' have "y ->β y' ∧ ys' = ys ∨ y' = y ∧ ys [->β]1 ys'" by simp
    hence "x # xs [->s] y' # ys'"
    proof
      assume H: "y ->β y' ∧ ys' = ys"
      with Cons' have "x ->s y'" by blast
      moreover from Cons' have "xs [->s] ys" by (iprover dest: listrelp_conj1)
      ultimately have "x # xs [->s] y' # ys" by (rule listrelp.Cons)
      with H show ?thesis by simp
    next
      assume H: "y' = y ∧ ys [->β]1 ys'"
      with Cons' have "x ->s y'" by blast
      moreover from H have "xs [->s] ys'" by (blast intro: Cons')
      ultimately show ?thesis by (rule listrelp.Cons)
    qed
    with ss show ?thesis by simp
  qed
qed

lemma lemma4:
  assumes r: "r ->s r'"
  shows "r' ->β r'' ==> r ->s r''" using r
proof (nominal_induct avoiding: r'' rule: sred.strong_induct)
  case (Var rs rs' x)
  then obtain ss where rs: "rs' [->β]1 ss" and r'': "r'' = Var x °° ss"
    by (blast dest: head_Var_reduction)
  from Var(1) [simplified] rs have "rs [->s] ss" by (rule lemma4_aux)
  hence "Var x °° rs ->s Var x °° ss" by (rule sred.Var)
  with r'' show ?case by simp
next
  case (Abs x ss ss' r r')
  from `(Lam [x].r') °° ss' ->β r''` show ?case
  proof (cases rule: apps_betasE [where x=x])
    case (appL s)
    then obtain r''' where s: "s = (Lam [x].r''')" and r''': "r' ->β r'''" using `x \<sharp> r''`
      by (cases rule: beta.strong_cases) (auto simp add: abs_fresh lam.inject alpha)
    from r''' have "r ->s r'''" by (blast intro: Abs)
    moreover from Abs have "ss [->s] ss'" by (iprover dest: listrelp_conj1)
    ultimately have "(Lam [x].r) °° ss ->s (Lam [x].r''') °° ss'" by (rule better_sred_Abs)
    with appL s show "(Lam [x].r) °° ss ->s r''" by simp
  next
    case (appR rs')
    from Abs(6) [simplified] `ss' [->β]1 rs'`
    have "ss [->s] rs'" by (rule lemma4_aux)
    with `r ->s r'` have "(Lam [x].r) °° ss ->s (Lam [x].r') °° rs'" by (rule better_sred_Abs)
    with appR show "(Lam [x].r) °° ss ->s r''" by simp
  next
    case (beta t u' us')
    then have Lam_eq: "(Lam [x].r') = (Lam [x].t)" and ss': "ss' = u' # us'"
      and r'': "r'' = t[x::=u'] °° us'"
      by (simp_all add: abs_fresh)
    from Abs(6) ss' obtain u us where
      ss: "ss = u # us" and u: "u ->s u'" and us: "us [->s] us'"
      by cases (auto dest!: listrelp_conj1)
    have "r[x::=u] ->s r'[x::=u']" using `r ->s r'` and u by (rule lemma3)
    with us have "r[x::=u] °° us ->s r'[x::=u'] °° us'" by (rule lemma1')
    hence "(Lam [x].r) ° u °° us ->s r'[x::=u'] °° us'" by (rule better_sred_Beta)
    with ss r'' Lam_eq show "(Lam [x].r) °° ss ->s r''" by (simp add: lam.inject alpha)
  qed
next
  case (Beta x s ss t r)
  show ?case
    by (rule better_sred_Beta) (rule Beta)+
qed

lemma rtrancl_beta_sred:
  assumes r: "r ->β* r'"
  shows "r ->s r'" using r
  by induct (iprover intro: refl_sred lemma4)+


subsection {* Terms in normal form *}

lemma listsp_eqvt [eqvt]:
  assumes xs: "listsp p (xs::'a::pt_name list)"
  shows "listsp ((pi::name prm) • p) (pi • xs)" using xs
  apply induct
  apply simp
  apply (rule listsp.intros)
  apply simp
  apply (rule listsp.intros)
  apply (drule_tac pi=pi in perm_boolI)
  apply perm_simp
  apply assumption
  done

inductive NF :: "lam => bool"
where
  App: "listsp NF ts ==> NF (Var x °° ts)"
| Abs: "NF t ==> NF (Lam [x].t)"

equivariance NF
nominal_inductive NF
  by (simp add: abs_fresh)

lemma Abs_NF:
  assumes NF: "NF ((Lam [x].t) °° ts)"
  shows "ts = []" using NF
proof cases
  case (App us i)
  thus ?thesis by (simp add: Var_apps_neq_Abs_apps [THEN not_sym])
next
  case (Abs u)
  thus ?thesis by simp
qed

text {*
@{term NF} characterizes exactly the terms that are in normal form.
*}
  
lemma NF_eq: "NF t = (∀t'. ¬ t ->β t')"
proof
  assume H: "NF t"
  show "∀t'. ¬ t ->β t'"
  proof
    fix t'
    from H show "¬ t ->β t'"
    proof (nominal_induct avoiding: t' rule: NF.strong_induct)
      case (App ts t)
      show ?case
      proof
        assume "Var t °° ts ->β t'"
        then obtain rs where "ts [->β]1 rs"
          by (iprover dest: head_Var_reduction)
        with App show False
          by (induct rs arbitrary: ts) (auto del: in_listspD)
      qed
    next
      case (Abs t x)
      show ?case
      proof
        assume "(Lam [x].t) ->β t'"
        then show False using Abs
          by (cases rule: beta.strong_cases) (auto simp add: abs_fresh lam.inject alpha)
      qed
    qed
  qed
next
  assume H: "∀t'. ¬ t ->β t'"
  then show "NF t"
  proof (nominal_induct t rule: Apps_lam_induct)
    case (1 n ts)
    then have "∀ts'. ¬ ts [->β]1 ts'"
      by (iprover intro: apps_preserves_betas)
    with 1(1) have "listsp NF ts"
      by (induct ts) (auto simp add: in_listsp_conv_set)
    then show ?case by (rule NF.App)
  next
    case (2 x u ts)
    show ?case
    proof (cases ts)
      case Nil
      from 2 have "∀u'. ¬ u ->β u'"
        by (auto intro: apps_preserves_beta)
      then have "NF u" by (rule 2)
      then have "NF (Lam [x].u)" by (rule NF.Abs)
      with Nil show ?thesis by simp
    next
      case (Cons r rs)
      have "(Lam [x].u) ° r ->β u[x::=r]" ..
      then have "(Lam [x].u) ° r °° rs ->β u[x::=r] °° rs"
        by (rule apps_preserves_beta)
      with Cons have "(Lam [x].u) °° ts ->β u[x::=r] °° rs"
        by simp
      with 2 show ?thesis by iprover
    qed
  qed
qed


subsection {* Leftmost reduction and weakly normalizing terms *}

inductive
  lred :: "lam => lam => bool"  (infixl "->l" 50)
  and lredlist :: "lam list => lam list => bool"  (infixl "[->l]" 50)
where
  "s [->l] t ≡ listrelp op ->l s t"
| Var: "rs [->l] rs' ==> Var x °° rs ->l Var x °° rs'"
| Abs: "r ->l r' ==> (Lam [x].r) ->l (Lam [x].r')"
| Beta: "r[x::=s] °° ss ->l t ==> (Lam [x].r) ° s °° ss ->l t"

lemma lred_imp_sred:
  assumes lred: "s ->l t"
  shows "s ->s t" using lred
proof induct
  case (Var rs rs' x)
  then have "rs [->s] rs'"
    by induct (iprover intro: listrelp.intros)+
  then show ?case by (rule sred.Var)
next
  case (Abs r r' x)
  from `r ->s r'`
  have "(Lam [x].r) °° [] ->s (Lam [x].r') °° []" using listrelp.Nil
    by (rule better_sred_Abs)
  then show ?case by simp
next
  case (Beta r x s ss t)
  from `r[x::=s] °° ss ->s t`
  show ?case by (rule better_sred_Beta)
qed

inductive WN :: "lam => bool"
  where
    Var: "listsp WN rs ==> WN (Var n °° rs)"
  | Lambda: "WN r ==> WN (Lam [x].r)"
  | Beta: "WN ((r[x::=s]) °° ss) ==> WN (((Lam [x].r) ° s) °° ss)"

lemma listrelp_imp_listsp1:
  assumes H: "listrelp (λx y. P x) xs ys"
  shows "listsp P xs" using H
  by induct auto

lemma listrelp_imp_listsp2:
  assumes H: "listrelp (λx y. P y) xs ys"
  shows "listsp P ys" using H
  by induct auto

lemma lemma5:
  assumes lred: "r ->l r'"
  shows "WN r" and "NF r'" using lred
  by induct
    (iprover dest: listrelp_conj1 listrelp_conj2
     listrelp_imp_listsp1 listrelp_imp_listsp2 intro: WN.intros
     NF.intros)+

lemma lemma6:
  assumes wn: "WN r"
  shows "∃r'. r ->l r'" using wn
proof induct
  case (Var rs n)
  then have "∃rs'. rs [->l] rs'"
    by induct (iprover intro: listrelp.intros)+
  then show ?case by (iprover intro: lred.Var)
qed (iprover intro: lred.intros)+

lemma lemma7:
  assumes r: "r ->s r'"
  shows "NF r' ==> r ->l r'" using r
proof induct
  case (Var rs rs' x)
  from `NF (Var x °° rs')` have "listsp NF rs'"
    by cases simp_all
  with Var(1) have "rs [->l] rs'"
  proof induct
    case Nil
    show ?case by (rule listrelp.Nil)
  next
    case (Cons x y xs ys)
    hence "x ->l y" and "xs [->l] ys" by (auto del: in_listspD)
    thus ?case by (rule listrelp.Cons)
  qed
  thus ?case by (rule lred.Var)
next
  case (Abs x ss ss' r r')
  from `NF ((Lam [x].r') °° ss')`
  have ss': "ss' = []" by (rule Abs_NF)
  from Abs(4) have ss: "ss = []" using ss'
    by cases simp_all
  from ss' Abs have "NF (Lam [x].r')" by simp
  hence "NF r'" by (cases rule: NF.strong_cases) (auto simp add: abs_fresh lam.inject alpha)
  with Abs have "r ->l r'" by simp
  hence "(Lam [x].r) ->l (Lam [x].r')" by (rule lred.Abs)
  with ss ss' show ?case by simp
next
  case (Beta x s ss t r)
  hence "r[x::=s] °° ss ->l t" by simp
  thus ?case by (rule lred.Beta)
qed

lemma WN_eq: "WN t = (∃t'. t ->β* t' ∧ NF t')"
proof
  assume "WN t"
  then have "∃t'. t ->l t'" by (rule lemma6)
  then obtain t' where t': "t ->l t'" ..
  then have NF: "NF t'" by (rule lemma5)
  from t' have "t ->s t'" by (rule lred_imp_sred)
  then have "t ->β* t'" by (rule lemma2)
  with NF show "∃t'. t ->β* t' ∧ NF t'" by iprover
next
  assume "∃t'. t ->β* t' ∧ NF t'"
  then obtain t' where t': "t ->β* t'" and NF: "NF t'"
    by iprover
  from t' have "t ->s t'" by (rule rtrancl_beta_sred)
  then have "t ->l t'" using NF by (rule lemma7)
  then show "WN t" by (rule lemma5)
qed

end