# kat_completeness: completeness of Kleene algebra with tests

We closely follow Dexter Kozen and Frederick Smith' proof: Kleene algebra with tests: Completeness and decidability. In Proc. CSL'96, vol. 1258 of LNCS, pages 244-259, 1996. Springer-Verlag.
The only difference is that we do the proof directly for typed KAT.
(We cannot easily exploit an untyping theorem, like we do in the case of KA: although the untyping theorem holds for KAT, it actually follows from typed completeness - at least we did not find an alternative way of proving it)
The proof can be summarised as follows: one exhibits a function hat: gregex n m gregex n m such that: 1. forall x, KAT |- hat x ≡ x 2. forall x, G(hat x) ≡ R(hat x), where . G(y) is the typed guarded strings interpretation of y . R(y) is the language interpretation of y, seen as a regular expression (the concrete coercions will be specified later on)
From these properties, it follows that for all x,y: gregex n m, we have G(x) ≡ G(y) => G(hat x) ≡ G(hat y) (1, and G is a model) => R(hat x) ≡ R(hat x) (2) => KA |- hat x ≡ hat y (KA completeness) => KA |- hat x ≡ hat y : n -> m (untyping theorem for KA) => KAT|- hat x ≡ hat y : n -> m (KA theorems hold in KAT) => KAT|- x ≡ y : n -> m (1, and transitivity)
(the converse is immediate, G being a model)
enable notations for positive
Require Import Coq.PArith.BinPosDef.

Require Import denum lset sums normalisation.
Require Import kat ka_completeness untyping.
Require Import regex gregex lsyntax syntax lang glang boolean atoms.
Set Implicit Arguments.

abbreviations: R is the module about regular expressions, while G is the module about generalised regular expressions
Module R := regex.
Module G := gregex.

Section KatCompleteness.
Notation Sigma := positive.
Variable pred: nat.
Variables s t: Sigma positive.
Notation gregex := (gregex_kat_ops pred s t).
Notation Atom := (ord (pow2 pred)).
Notation gword := (trace Atom).
Notation glang := (tglang_kat_ops pred s t).
Notation g_atom n a := (@g_prd pred s t n (@atom pred a)).
Notation test := (lsyntax.expr (ord pred)).
Local Open Scope list_scope.

# 1. Definition of the hat function, and correctness

## externally guarded terms

the hat function is defined by induction on the structure of its argument, but it actually produces formal sums of "externally guarded terms", defined by the following inductive

Inductive guard: positive positive Type :=
| g_pred: {n} (a: Atom), guard n n
| g_elem: n m (a: Atom) (e: gregex n m) (b: Atom), guard n m.
Notation guards n m := (list (guard n m)).

externally guarded terms, and sums of such terms can be converted back to gregex in the obvious way
Definition geval n m (x: guard n m) :=
match x with
| @g_pred n ag_atom n a
| @g_elem n m a e bg_atom n a e g_atom m b
end.
Notation teval := (sup (@geval _ _)).

## inductive cases for the hat function

### predicates

a predicate is mapped to the set of atoms under which it is satisfied
Definition g_prd' n p: guards n n :=
map g_pred (filter (fun alsyntax.eval (set.mem a) p) (seq (pow2 pred))).

Lemma teval_prd n (p: tst n): teval (g_prd' n p) inj p.
Proof.
unfold g_prd'. rewrite sup_map. unfold geval.
setoid_rewrite <-inj_sup. apply inj_weq.
symmetry. apply decomp_expr.
Qed.

### unit

accordingly, 1 is simply mapped to the set of all atoms
Definition g_one' n := g_prd' n (@lsyntax.e_top _).

Lemma teval_one n: teval (g_one' n) 1.
Proof. unfold g_one'. rewrite teval_prd. apply inj_top. Qed.

### letters

a Kleene variable i is mapped to the sum of all fig, for f,g arbitrary atoms

Definition g_var' i := \sup_(f<_) \sup_(g<_) [g_elem f (g_var i) g]%list.

Lemma sum_atoms n: \sup_(i<pow2 pred) g_atom n i 1.
Proof.
rewrite <- teval_one. unfold g_one', g_prd'. rewrite sup_map.
apply sup_weq. reflexivity.
induction seq. reflexivity. now apply (cup_weq [_] [_])%list.
Qed.

Lemma teval_var i: teval (g_var' i) g_var i.
Proof.
unfold g_var'. rewrite sup_sup. setoid_rewrite sup_sup.
setoid_rewrite sup_singleton. unfold geval.
setoid_rewrite <-dotxsum. setoid_rewrite <-dotA. rewrite <-dotsumx.
setoid_rewrite sum_atoms. ra.
Qed.

### composition

composition is defined by a kind of coalesced product: we take all products such that the post-guard of the former element coincides with the pre-guard of the latter
g_dot1 x y tries to compose two externally guarded terms
Definition g_dot1 n m (x: guard n m): p, guard m p guards n p :=
match x with
| g_pred afun p y
match y with
| g_pred bif eqb a b then [g_pred a] else []
| g_elem b e cif eqb a b then [g_elem b e c] else []
end
| g_elem a e bfun p y
match y with
| g_pred cfun eif eqb b c then [g_elem a e b] else []
| g_elem c f dfun eif eqb b c then [g_elem a (eg_atom _ bf) d] else []
end e
end%list.

g_dot' h k does the composition of two lists of externally guarded terms
Definition g_dot' n m p (h: guards n m) (k: guards m p) :=
\sup_(x\in h) \sup_(y\in k) g_dot1 x y.

the correctness of this construction relies on the following two lemma
Lemma empty_atom_dot n a b: ab g_atom n a g_atom n b 0.
Proof.
intros. setoid_rewrite <-inj_cap.
rewrite (empty_atom_cap H). apply inj_bot.
Qed.

Lemma idem_atom_dot n a: g_atom n a g_atom n a g_atom n a.
Proof. setoid_rewrite <-inj_cap. now rewrite capI. Qed.

correctness of g_dot1
Lemma geval_dot n m p (x: guard n m) (y: guard m p): teval (g_dot1 x y) geval x geval y.
Proof.
destruct x as [? a|? ? a e b]; destruct y as [? c|? ? c f d]; unfold g_dot1;
(case eqb_spec; [intros <-|intro E]); unfold geval; rewrite ?sup_singleton; unfold sup.
- now rewrite idem_atom_dot.
- symmetry. apply (empty_atom_dot _ E).
- now rewrite 2dotA, idem_atom_dot.
- rewrite 2dotA, (empty_atom_dot _ E). ra.
- now rewrite <-3dotA, idem_atom_dot.
- rewrite <-2dotA, (empty_atom_dot _ E). ra.
- transitivity (g_atom _ a⋅(e⋅(g_atom _ bg_atom _ b)⋅f)⋅g_atom _ d). 2:ra.
now rewrite idem_atom_dot.
- transitivity (g_atom _ a⋅(e⋅(g_atom _ bg_atom _ c)⋅f)⋅g_atom _ d). 2:ra.
rewrite (empty_atom_dot _ E). ra.
Qed.

correctness of g_dot'
Lemma teval_dot n m p (x: guards n m) (y: guards m p):
teval (g_dot' x y) teval x teval y.
Proof.
unfold g_dot'. rewrite sup_sup. setoid_rewrite sup_sup.
setoid_rewrite geval_dot.
rewrite dotsumx. now setoid_rewrite dotxsum.
Qed.

### Kleene star

Kleene star is defined by induction on the list of externally guarded terms, see Kozen and Smith' paper

Definition fst n m (x: guard n m) := match x with g_pred a | g_elem a _ _a end.
Definition lst n m (x: guard n m) := match x with g_pred a | g_elem _ _ aa end.
Definition g_inner_dot n m (x: guard n m): p, guard m p gregex n p :=
match x with
| g_pred afun p y
match y with
| g_pred b ⇒ 0
| g_elem b e cif eqb a b eqb a c then e else 0
end
| g_elem a e bfun p y
match y with
| g_pred cfun eif eqb a b eqb b c then e else 0
| g_elem c f dfun eif eqb b c eqb a d then eg_atom _ bf else 0
end e
end.

Definition xitr n m (r: guard n m) q' :=
let rq' := g_dot' [r] q' in
let a := fst r in
let p := sup (@g_inner_dot _ _ r _) q' in
g_dot' ([g_elem a (p(g_atom _ ap)^*) a]++g_one' _) rq'.

Fixpoint g_str' n (x: guards n n) :=
match x with
| []g_one' _
| r::q
let q' := g_str' q in
q' ++ g_dot' q' (xitr r q')
end.

the correctness of this construction is substantially more involved than for the other ones

Lemma geval_fst n m (r: guard n m): geval r g_atom _ (fst r) geval r.
Proof.
destruct r.
simpl fst; unfold geval. now rewrite idem_atom_dot.
simpl fst; unfold geval. now rewrite 2dotA, idem_atom_dot.
Qed.

Lemma geval_lst n m (r: guard n m): geval r geval r g_atom _ (lst r).
Proof.
destruct r.
simpl fst; unfold geval. now rewrite idem_atom_dot.
simpl fst; unfold geval. now rewrite <-3dotA, idem_atom_dot.
Qed.

Definition dirac n m: gregex n m.
case (eqb_pos_spec n m). intros <-. exact 1. intros _. exact 0.
Defined.

Lemma dirac_refl n: dirac n n = 1.
Proof.
unfold dirac. case eqb_pos_spec. 2: congruence.
intro. now rewrite cmp_eq_rect_eq.
Qed.

Lemma teval_inner_dot n m p (x: guard n m) (y: guard m p):
dirac n p + g_atom _ (fst x) g_inner_dot x y g_atom _ (fst x)
dirac n p + ofbool (eqb (fst x) (lst y)) geval x geval y.
Proof.
unfold g_inner_dot.
revert p y. destruct x as [n a|n m a e b]; destruct y as [p c|p q c f d];
simpl fst; simpl lst; simpl geval.
rewrite dirac_refl. case eqb. 2: ra. ra_normalise.
setoid_rewrite <-inj_cap. rewrite <-inj_top, <-inj_cup. apply inj_weq. lattice.
case eqb_spec; intro E. subst. case eqb_spec; intro E'. subst.
simpl. ra_normalise. now rewrite idem_atom_dot.
ra.
simpl. ra_normalise. rewrite <-(dotA _ (g_atom p a)).
rewrite empty_atom_dot by assumption. ra.
case eqb_spec; intro E. subst. case eqb_spec; intro E'. subst.
simpl. ra_normalise. now rewrite <-3dotA, idem_atom_dot.
ra.
simpl. case eqb_spec; intro E'. subst. ra_normalise.
rewrite <-dotA. rewrite empty_atom_dot by congruence. ra.
ra.
case eqb_spec; intro E. subst. case eqb_spec; intro E'. subst.
simpl. ra_normalise. now rewrite <-(dotA _ (g_atom p c) (g_atom p c)), idem_atom_dot.
ra.
simpl. case eqb_spec; intro E'. subst.
ra_normalise. rewrite <-(dotA _ (g_atom p b) (g_atom p c)), empty_atom_dot by assumption.
ra.
ra.
Qed.

Lemma teval_xitr n m (r: guard n m) q: teval (xitr r q) (geval r teval q)^+.
Proof.
unfold xitr.
rewrite 2teval_dot, sup_app, 2sup_singleton, teval_one.
symmetry. rewrite itr_str_l. rewrite (geval_fst r) at 2.
rewrite <-(dotA (g_atom _ _)), str_dot.
apply dot_weq. 2: reflexivity.
rewrite cupC. unfold geval at 3. rewrite dotA. rewrite <-itr_str_l.
rewrite itr_aea by now rewrite idem_atom_dot. rewrite <-str_itr. apply str_weq1.
induction q as [|e q IH]. ra.
simpl (sup _ _).
rewrite 2dotxpls, 2dotplsx. rewrite <-(cupI 1), 2cupA, 2comm4. apply cup_weq.
2: assumption. clear IH.
rewrite <-dirac_refl. rewrite teval_inner_dot.
case eqb_spec; intro E.
setoid_rewrite dot1x. rewrite E. now rewrite <-dotA, <-geval_lst.
rewrite (geval_lst e), <-2dotA, empty_atom_dot by congruence. ra.
Qed.

Lemma teval_str n (x: guards n n): teval (g_str' x) (teval x)^*.
Proof.
induction x as [|r q IH]; simpl g_str'.
rewrite teval_one. symmetry. apply str0.
simpl (sup _ _). setoid_rewrite (cupC (geval r)). rewrite str_pls.
rewrite sup_app, teval_dot, teval_xitr, IH. rewrite (str_itr (__)). ra.
Qed.

## summing up the constructions, by induction

Fixpoint hat n m (e: gregex n m): guards n m :=
match e with
| g_zer _ _ _[]
| g_prd _ _ pg_prd' _ p
| g_pls e fhat e hat f
| g_dot e fg_dot' (hat e) (hat f)
| g_itr eg_dot' (hat e) (g_str' (hat e))
| g_var ig_var' i
end.

Theorem teval_hat n m (e: gregex n m): teval (hat e) e.
Proof.
induction e; simpl hat.
reflexivity.
apply teval_prd.
apply teval_var.
setoid_rewrite sup_app. now apply cup_weq.
rewrite teval_dot. now apply dot_weq.
rewrite teval_dot, teval_str, <-itr_str_l. now apply itr_weq.
Qed.

# Relationship between generalised regular expressions and (plain) regular expressions

## extended alphabet

a letter in the extended alphabet is either a Kleene variable, a positive predicate variable, or a negative one. We moreover need to record the type of the corresponding test in the two latter cases

Inductive letter :=
| l_pos (n: positive) (p: ord pred)
| l_neg (n: positive) (p: ord pred)
| l_var (i: Sigma).

the above type can be retracted into positives: this saves us from proving KA completeness on an arbitrary alphabet (this would require a lot of polymorphic definitions)

Definition lp (l: letter): positive :=
match l with
| l_pos n xmk_sum (inl (mk_sum (inl (mk_pair (n, mk_ord x)))))
| l_neg n xmk_sum (inl (mk_sum (inr (mk_pair (n, mk_ord x)))))
| l_var imk_sum (inr i)
end.
Definition pl (p: positive): letter :=
match get_sum p with
| inl pmatch get_sum p with
| inl plet '(n,p) := get_pair p in
match get_ord _ p with Nonel_var 1 | Some xl_pos n x end
| inr plet '(n,p) := get_pair p in
match get_ord _ p with Nonel_var 1 | Some xl_neg n x end
end
| inr il_var i
end.
Lemma plp l: pl (lp l) = l.
Proof. destruct l; unfold pl, lp; now rewrite !get_mk_sum, ?get_mk_pair, ?get_mk_ord. Qed.

the retraction into positives also equips letter with a cmpType structure
Definition compare_letter (a b: letter) := cmp (lp a) (lp b).
Lemma compare_letter_spec a b: compare_spec (a=b) (compare_letter a b).
Proof.
unfold compare_letter. case cmp_spec; intro E; constructor.
now rewrite <-(plp a), <-(plp b), E. congruence. congruence.
Qed.
Canonical Structure cmp_letter := mk_simple_cmp _ compare_letter_spec.

typing extended letters:
• predicate letters come with their type
• Kleene letters use the typing environment
Definition src' l :=
match l with
| l_pos n _ | l_neg n _n
| l_var is i
end.

Definition tgt' l :=
match l with
| l_pos n _ | l_neg n _n
| l_var it i
end.

## regular expressions on the extended alphabet

expr3 is intuitively the set of typed regular expressions on letter, while uexpr3 is the set of untyped regular expressions on letter
we use uexpr3 rather than regex to use the untyping theorem (which we proved on generic expressions rather than regular expressions).
we now define several maps between these representations:
and we prove the following properties:
• o'o = id (yielding injectivity of o)
• wvo = uo (allowing us to use the untyping theorem)
Notation expr3 n m := (expr_ops src' tgt' BKA n m).
Notation uexpr3 := (expr_ops (fun _xH) (fun _xH) BKA xH xH).

## o:gregexnm→expr3nm

Section n.
Variable n: positive.
Import lsyntax.
we need to push negation to leaves in Boolean expressions
Fixpoint o_pred (x: test): expr3 n n :=
match x with
| e_bot ⇒ 0
| e_top ⇒ 1
| e_cup x yo_pred x + o_pred y
| e_cap x yo_pred x o_pred y
| e_neg xo_npred x
| e_var asyntax.e_var (l_pos n a)
end
with o_npred (x: test): expr3 n n :=
match x with
| e_bot ⇒ 1
| e_top ⇒ 0
| e_cup x yo_npred x o_npred y
| e_cap x yo_npred x + o_npred y
| e_neg xo_pred x
| e_var asyntax.e_var (l_neg n a)
end.
Import syntax.
Lemma o_pred_level x: e_level (o_pred x) BKA
with o_npred_level x: e_level (o_npred x) BKA.
Proof.
destruct x; simpl o_pred; simpl e_level; rewrite ?merge_spec; intuition.
destruct x; simpl o_npred; simpl e_level; rewrite ?merge_spec; intuition.
Qed.
End n.

Fixpoint o n m (e: gregex n m): expr3 n m:=
match e with
| g_zer _ _ _ ⇒ 0
| g_prd _ _ po_pred _ p
| g_pls e fo e + o f
| g_dot e fo e o f
| g_itr e(o e)^+
| g_var je_var (l_var j)
end.

Lemma o_sup n m I J (f: I gregex n m): o (sup f J) = \sup_(i\in J) (o (f i)).
Proof. apply f_sup_eq; now f_equal. Qed.

Lemma o_level n m (e: gregex n m): e_level (o e) BKA.
Proof.
pose proof o_pred_level.
induction e; simpl o; simpl e_level; rewrite ?merge_spec; intuition.
Qed.

## o':expr3nm→gregexnm

Definition o': n m, expr3 n m gregex n m :=
@eval _ src' tgt' (gregex_monoid_ops pred s t) id
(fun lmatch l return gregex (src' l) (tgt' l) with
| l_pos n pg_prd _ _ (lsyntax.e_var p)
| l_neg n pg_prd _ _ (! lsyntax.e_var p)
| l_var ig_var i
end).

Lemma o'o_pred: n (a: test), o' (o_pred n a) g_prd _ _ a
with o'o_npred: n (a: test), o' (o_npred n a) g_prd _ _ (!a).
Proof.
destruct a.
symmetry. apply inj_bot.
symmetry. apply inj_top.
setoid_rewrite inj_cup. apply cup_weq; apply o'o_pred.
setoid_rewrite inj_cap. apply dot_weq; apply o'o_pred.
apply o'o_npred.
reflexivity.
destruct a.
symmetry. etransitivity. apply inj_weq. apply negbot. apply inj_top.
symmetry. etransitivity. apply inj_weq. apply negtop. apply inj_bot.
etransitivity. 2: apply inj_weq; symmetry; apply negcup. rewrite inj_cap.
apply dot_weq; apply o'o_npred.
etransitivity. 2: apply inj_weq; symmetry; apply negcap. rewrite inj_cup.
apply cup_weq; apply o'o_npred.
etransitivity. 2: apply inj_weq; symmetry; apply negneg. apply o'o_pred.
reflexivity.
Qed.

Lemma o'o: n m (e: gregex n m), o' (o e) e.
Proof.
induction e; simpl o; simpl o'.
reflexivity.
apply o'o_pred.
reflexivity.
now apply cup_weq.
now apply dot_weq.
now apply itr_weq.
Qed.

Lemma o'_weq n m: Proper (weq ==> weq) (@o' n m).
Proof. intros ? ? H. apply (H (gregex_monoid_ops _ _ _) _ id). Qed.

so that o is injective
Corollary o_inj n m (e f: gregex n m): o e o f e f.
Proof. intro H. apply o'_weq in H. revert H. now rewrite 2o'o. Qed.

## expr3-v→regex-w→uexpr3

Definition v: n m, expr3 n m regex :=
eval (f':=fun _regex_tt) (fun lr_var (lp l)).

Definition w (e: regex): uexpr3 :=
eval (X:=expr_ops _ _ BKA) (f':=fun _xH) (fun pe_var (pl p)) (to_expr e).

Lemma wv_u n m (e: expr3 n m): e_level e BKA w (v e) erase BKA e.
Proof.
unfold w.
induction e; simpl e_level; intro Hl; try discriminate_levels;
try (first [reflexivity|apply dot_weq|apply cup_weq|apply str_weq]);
try (first [apply IHe1|apply IHe2|apply IHe]; solve_lower').
symmetry. simpl (erase _ _). rewrite itr_str_l, <-IHe. reflexivity. solve_lower'.
simpl. now rewrite plp.
Qed.

key lemma to be able to use the untyping theorem
Lemma wvo_uo n m (e: gregex n m): w (v (o e)) erase BKA (o e).
Proof. apply wv_u, o_level. Qed.
Lemma w_weq: Proper (weq ==> weq) w.
Proof. intros ? ? H. apply (H (expr_ops _ _ _) _ (fun _xH)). Qed.

Lemma v_sup n m I J (f: I expr3 n m): v (sup f J) = \sup_(i\in J) (v (f i)).
Proof. apply f_sup_eq; now f_equal. Qed.

# From guarded string languages to languages on the extended alphabet

i.e., we define a coercion from glang lang

Notation word := (list positive).
Notation lang := (lang_ops positive lang_tt lang_tt).

converting an atom into a word of the extended alphabet. this word has length pred: each predicate variable appears exactly once, with its assigned truth value
Definition atom_to_word n (a: Atom) :=
map (fun iif set.mem a i then lp (l_pos n i) else lp (l_neg n i)) (seq pred).

converting an guarded string into a word of the extended alphabet the resulting word has length pred+(1+pred)⋅n, where n is the length of the guarded string
Fixpoint gword_to_word n (w: gword) :=
match w with
| tnil aatom_to_word n a
| tcons a i watom_to_word n a ++ lp (l_var i) :: gword_to_word (t i) w
end.

we convert a guarded string language by converting its words
Definition gl n m (G: glang n m): lang :=
fun w g, w = gword_to_word n g proj1_sig G g.

Instance gl_leq n m: Proper (leq ==> leq) (@gl n m).
Proof. intros G G' H w [v [? Hv]]. v. split. assumption. apply H, Hv. Qed.
Instance gl_weq n m: Proper (weq ==> weq) (@gl n m) := op_leq_weq_1.

auxiliary definition for the following auxiliary lemma: gword_to_word' w return the suffix of the word corresponding to w, where the initial atom has been omitted
Definition gword_to_word' (w: gword) :=
match w with
| tnil a[]
| tcons a i wlp (l_var i) :: gword_to_word (t i) w
end.

Lemma gword_to_word_cut n w:
gword_to_word n w = atom_to_word n (thead w) ++ gword_to_word' w.
Proof. destruct w. apply app_nil_end. reflexivity. Qed.

Lemma gword_tapp: x y z, tapp x y z
n, gword_to_word n z = gword_to_word n x ++ gword_to_word' y.
Proof.
induction 1; simpl; intros n.
apply app_nil_end.
reflexivity.
now rewrite IHtapp, app_ass.
Qed.

# 2. G (hat e) = R (hat e)

(more formally, gl (G (hat e)) ≡ R (v (o (hat e))))

Notation R := R.lang.
Notation G := G.lang.

Notation latom n a := (eq (atom_to_word n a): lang).

regular language corresponding to an atom

Lemma R_lang_atom n a: R (v (o (g_atom n a))) latom n a.
Proof.
simpl o. unfold atom, atom_to_word.
induction (seq pred). apply R.lang_1. simpl (sup _ _).
setoid_rewrite R.lang_dot. setoid_rewrite (eq_app_dot _ [_]). apply dot_weq.
case set.mem; apply R.lang_var. assumption.
Qed.

## properties of gl

gl is a semilattice morphism (note that it does not preserve products/iterations)

Lemma gl_bot n m: @gl n m bot bot.
Proof. split. intros [? [? []]]. intros []. Qed.

Lemma gl_cup n m (e f: glang n m): gl (e f) gl e gl f.
Proof.
split.
intros [w [-> [H|H]]]; [left|right]; w; split; auto.
intros [H|H]; destruct H as [w [-> H]]; w; split; trivial; (now left) || (now right).
Qed.

Lemma gl_sup n m I J (f: I glang n m): gl (sup f J) \sup_(i\in J) gl (f i).
Proof. apply f_sup_weq. apply gl_bot. apply gl_cup. Qed.

gl maps atoms to atoms

Lemma gl_atom n a: gl (tatom n a) latom n a.
Proof.
intro w; split.
intros [g [-> <-]]. reflexivity.
intros <-. eexists. split. 2: reflexivity. reflexivity.
Qed.

image of single letter traces under gl
Lemma gl_single' a i b:
gl (tsingle' a i b)
eq (atom_to_word (s i) a++[lp (l_var i)]++atom_to_word (t i) b).
Proof.
intro w; split.
intros [g [-> <-]]. reflexivity.
intros <-. eexists. split. 2: reflexivity. reflexivity.
Qed.

key auxiliary lemma for composition: we need to use an atom as a cutting point
Lemma gl_dot n m p (e: glang n m) a (f: glang m p) (e' f': lang):
gl (e tatom m a) e' latom m a
gl (tatom m a f) latom m a f'
gl (e tatom m a f) e' latom m a f'.
Proof.
setoid_rewrite weq_spec.
intros He Hf. split; intro w.
- apply proj1 in He. apply proj1 in Hf.
intros [g [-> [xa [x Hx [? <- Hxa]] [y Hy Hg]]]].
apply tapp_x_nil_eq in Hxa as [Ha ->].
destruct (tapp_bounds Hg) as (H1&H2&H3).     destruct (He (gword_to_word n x)) as [x' Hx' [? <- Hxa']].
repeat eexists; eauto. rewrite Ha. apply tapp_x_nil.
destruct (Hf (gword_to_word m y)) as [? <- [y' Hy' Hay']].
repeat eexists; eauto. rewrite Ha. rewrite H1. apply tapp_nil_x.
repeat eexists; eauto.
rewrite app_ass, <-Hay', (gword_to_word_cut m y), (gword_tapp Hg), <-app_ass.
congruence.
- apply proj2 in He. apply proj2 in Hf.
intros [ea [x Hx [? <- ->]] [y Hy ->]].
edestruct (He (x++atom_to_word m a)) as [xa [Hxa [x' Hx' [? <- Hxa']]]]. eexists; eauto.
edestruct (Hf (atom_to_word m a++y)) as [ay [Hay [? <- [y' Hy' Hay']]]]. eexists; eauto.
apply tapp_x_nil_eq in Hxa' as [-> ->].
apply tapp_nil_x_eq in Hay' as [Ha ->].
destruct (tapp_cat x' y' Ha) as [z Hz].
repeat eexists; eauto. 2: apply tapp_x_nil.
rewrite (gword_tapp Hz), <-Hxa, app_ass, Hay, gword_to_word_cut, ass_app.
congruence.
Qed.

Lemma gl_nildot a n m (e: glang n m): e', gl (tatom n a e) latom n a e'.
Proof.
(fun w g, proj1_sig e g thead g = a w = gword_to_word' g).
rewrite weq_spec. split; intro w.
intros [u [-> [? <- [v Hv H]]]]. apply tapp_nil_x_eq in H as [-> ->].
repeat eexists; eauto. apply gword_to_word_cut.
intros [? <- [u (x&He&<-&->) ->]].
repeat eexists; eauto. apply eq_sym, gword_to_word_cut. apply tapp_nil_x.
Qed.

key auxiliary lemma for iteration: we need to use an atom as bounds
Lemma gl_itr n (e: glang n n) a (e': lang):
gl (tatom n a e tatom n a) e' latom n a
gl ((tatom n a e)^+ tatom n a) e'^+ latom n a.
Proof.
rename n into m.
intro H. rewrite <-itr_dot. apply antisym.
- apply weq_spec in H as [H _]. intros w [g [-> [? <- [u [n Hn] Hg]]]].
apply tapp_nil_x_eq in Hg as [Hu ->]. revert u Hu Hn. induction n; intros u Hu Hn.
destruct Hn as [v Hv [? <- Hn]]. apply tapp_x_nil_eq in Hn as [Hn ->].
destruct (H (gword_to_word m v)) as [u He [? <- H']].
repeat eexists; eauto. rewrite Hu. apply tapp_nil_x. rewrite Hn. apply tapp_x_nil.
u. now O. eauto.
destruct Hn as [w [v Hv [? <- Hn]] [w' Hw' H']].
apply tapp_x_nil_eq in Hn as [Hn ->].
assert (H'' := tapp_bounds H').     assert (Haw: a=thead w') by (rewrite Hn; intuition congruence).
destruct (H (gword_to_word m v)) as [xe He [? <- Hxe]].
repeat eexists; eauto.
rewrite Hu, <-(tapp_head H'). apply tapp_nil_x.
rewrite Hn; apply tapp_x_nil.
assert (Hext: e' (e'^+ latom m a) e'^+ latom m a).
now rewrite dotA, itr_cons.
apply Hext. clear Hext.
eexists. eassumption. eexists. apply IHn. apply Haw.
assumption.
rewrite (gword_tapp H'), Hxe, gword_to_word_cut, Haw, app_ass. congruence.
- apply itr_ind_l.
now rewrite <-H, <-itr_ext, dotA.
rewrite <-itr_cons at 2.
destruct (gl_nildot a ((e tatom m a)^+)) as [f' Hf].
rewrite 2dotA. rewrite gl_dot by eassumption.
now rewrite Hf, dotA.
Qed.

## clean terms: those on which G and R coincide

Definition clean1 n m (e: guard n m) := gl (G (geval e)) R (v (o (geval e))).
Definition clean n m (e: guards n m) := g, In g e clean1 g.

Lemma G_clean n m (e: guards n m): clean e gl (G (teval e)) R (v (o (teval e))).
Proof.
intro He. rewrite o_sup, v_sup, lang_sup, R.lang_sup, gl_sup.
apply sup_weq'. reflexivity. intros g Hg. apply He, Hg.
Qed.

basic constructors for clean terms

Lemma clean_bot n m: @clean n m bot.
Proof. intros _ []. Qed.

Lemma clean_cup n m (e f: guards n m): clean e clean f clean (e++f).
Proof. unfold clean. setoid_rewrite in_app_iff. intuition. Qed.

Lemma clean_single n m (g: guard n m): clean1 g clean [g].
Proof. now intros ? ? [<-|[]]. Qed.

Lemma clean_sup n m I J (f: I guards n m): ( i, In i J clean (f i)) clean (sup f J).
Proof. apply P_sup. apply clean_bot. apply clean_cup. Qed.

Lemma clean_map n m I J (f: I guard n m): ( i, In i J clean1 (f i)) clean (map f J).
Proof. rewrite map_sup. intro. apply clean_sup. intros. apply clean_single. auto. Qed.

## the basic ingredients of the hat function preserve cleaness

Lemma clean_pred n a: clean1 (@g_pred n a).
Proof. unfold clean1, geval. rewrite G.lang_atom, R_lang_atom. apply gl_atom. Qed.

Lemma clean_elem_var a i b: clean1 (g_elem a (g_var i) b).
Proof.
unfold clean1, geval.
rewrite 2G.lang_dot, 2G.lang_atom.
setoid_rewrite atom_single_atom. rewrite gl_single'. rewrite 2eq_app_dot.
do 2 setoid_rewrite R.lang_dot. setoid_rewrite R.lang_var.
setoid_rewrite <- R_lang_atom. apply dotA.
Qed.

Lemma clean_dot' n m p (e: guards n m) (f: guards m p): clean e clean f clean (g_dot' e f).
Proof.
intros He Hf. apply clean_sup. intros x Hx. apply clean_sup. intros y Hy.
apply He in Hx. apply Hf in Hy. clear e f He Hf.
destruct x as [a|a e b]; destruct y as [c|c f d]; unfold g_dot1;
(case eqb_spec; [intros <-|intro E]); try apply clean_bot; apply clean_single; trivial.
revert Hx Hy. unfold clean1, geval. rewrite 8G.lang_dot, 3G.lang_atom.
simpl o; simpl v. fold_regex. rewrite 8R.lang_dot. repeat change (o_pred ?a (atom ?b)) with (o (g_atom a b)). setoid_rewrite R_lang_atom.
intros Hx Hy. rewrite 4dotA, <-dotA, <-(dotA _ _ (latom _ _)).
apply gl_dot. assumption. rewrite 2dotA. assumption.
Qed.

Lemma clean_one' n: clean (g_one' n).
Proof. apply clean_map. intros. apply clean_pred. Qed.

Lemma clean_inner_dot n m (e: guard n m) (He: clean1 e):
p (f: guard m p), clean1 f
gl (tatom n (fst e) G (g_inner_dot e f) tatom p (fst e))
latom n (fst e) R (v (o (g_inner_dot e f))) latom p (fst e).
Proof.
assert (Z: n m p q (e: glang n m) (f: glang p q) e' f', gl (eG 0f) e'R 0f').
intros. rewrite G.lang_0, dotx0, dot0x, gl_bot. setoid_rewrite R.lang_0. ra.
destruct e as [n a|n m a e b]; destruct f as [p c|p q c f d]; intros Hf;
simpl fst; simpl lst; simpl g_inner_dot.
- apply Z.
- case eqb_spec. intros <-. 2: intros; apply Z.
case eqb_spec. intros <-. 2: intros; apply Z.
unfold andb. unfold clean1, geval in Hf.
rewrite 2G.lang_dot, 2G.lang_atom in Hf. rewrite Hf.
rewrite <-2R_lang_atom, <-2R.lang_dot. reflexivity.
- case eqb_spec. intros <-. 2: intros; apply Z.
case eqb_spec. intros <-. 2: intros; apply Z.
unfold andb. unfold clean1, geval in He.
rewrite 2G.lang_dot, 2G.lang_atom in He. rewrite He.
rewrite <-2R_lang_atom, <-2R.lang_dot. reflexivity.
- case eqb_spec. intros <-. 2: intros; apply Z.
case eqb_spec. intros <-. 2: intros; apply Z.
unfold andb.
unfold clean1, geval in He. rewrite 2G.lang_dot, 2G.lang_atom in He.
unfold clean1, geval in Hf. rewrite 2G.lang_dot, 2G.lang_atom in Hf.
simpl o in *; simpl v in ×.
rewrite 2R.lang_dot in He. repeat change (o_pred ?a (atom ?b)) with (o (g_atom a b)) in He. setoid_rewrite R_lang_atom in He.
rewrite 2R.lang_dot in Hf. repeat change (o_pred ?a (atom ?b)) with (o (g_atom a b)) in Hf. setoid_rewrite R_lang_atom in Hf.
rewrite 2R.lang_dot. repeat change (o_pred ?a (atom ?b)) with (o (g_atom a b)). setoid_rewrite R_lang_atom.
setoid_rewrite G.lang_dot. rewrite dotA. setoid_rewrite G.lang_dot.
rewrite G.lang_atom.
rewrite dotA. rewrite <-(dotA _ (G f)).
rewrite <-2dotA in Hf.
rewrite gl_dot by eassumption. ra.
Qed.

Lemma clean_str' n (e: guards n n): clean e clean (g_str' e).
Proof.
induction e; intro Hae; simpl g_str'. apply clean_one'.
assert (He: clean e) by (intros ? ?; apply Hae; now right). specialize (IHe He).
assert (Ha: clean1 a) by (apply Hae; now left). clear Hae.
apply clean_cup. assumption.
apply clean_dot'. assumption.
apply clean_dot'. apply clean_cup. 2: apply clean_one'.
2: apply clean_dot'; [now apply clean_single | assumption].
revert IHe. generalize (g_str' e). clear e He. intros e He.
apply clean_single. unfold clean1. unfold geval. simpl o; simpl v; fold_regex.
rewrite 2dotA, <-str_itr, <-2itr_str_l.
rewrite G.lang_dot, G.lang_itr, G.lang_dot, G.lang_sup, G.lang_atom.
rewrite R.lang_dot, R.lang_itr, R.lang_dot, o_sup, v_sup, R.lang_sup.
repeat change (o_pred ?a (atom ?b)) with (o (g_atom a b)). setoid_rewrite R_lang_atom.
apply gl_itr. rewrite 2dotxsum, 2dotsumx, gl_sup.
apply sup_weq'. reflexivity. intros f Hf.
apply clean_inner_dot. apply Ha. apply He, Hf.
Qed.

## the hat function produces clean terms

Theorem clean_hat n m (e: gregex n m): clean (hat e).
Proof.
induction e; simpl hat.
apply clean_bot.
apply clean_map. intros ? _. apply clean_pred.
apply clean_sup. intros ? _. apply clean_sup. intros ? _. apply clean_single, clean_elem_var.
now apply clean_cup.
now apply clean_dot'.
apply clean_dot'. assumption. apply clean_str'. assumption.
Qed.

whence the desired property, as a corollary

Corollary G_hat n m (e: gregex n m): gl (G e) R (v (o (teval (hat e)))).
Proof. rewrite <-(teval_hat e) at 1. apply G_clean, clean_hat. Qed.

# KAT completeness

we assemble all the pieces as explained at the beginning of this file
Theorem kat_complete_weq n m: e f: gregex n m, G e G f e f.
Proof.
intros e f H.
apply gl_weq in H.
rewrite 2G_hat in H.
apply ka_complete_weq in H.
apply w_weq in H.
rewrite 2wvo_uo in H.
apply erase_faithful_weq in H.
2: now rewrite 2o_level.
2: reflexivity.
apply o_inj in H.
rewrite 2teval_hat in H.
assumption.
Qed.

we deduce a similar result for language inclusions
Corollary kat_complete_leq n m: e f: gregex n m, G e G f e f.
Proof. intros e f. rewrite 2leq_iff_cup, <-G.lang_pls. apply kat_complete_weq. Qed.

and the above implications actually are equivalences
Corollary kat_correct_complete_weq n m: e f: gregex n m, G e G f e f.
Proof. split. apply kat_complete_weq. apply G.lang_weq. Qed.

Corollary kat_correct_complete_leq n m: e f: gregex n m, G e G f e f.
Proof. split. apply kat_complete_leq. apply G.lang_leq. Qed.

# KAT decidability

• KAT proofs reduce to KA proofs
• KAT equality is decidable

Corollary kat_reduces_to_ka n m: e f: gregex n m,
e f v (o (teval (hat e))) v (o (teval (hat f))).
Proof.
intros. split; intro H.
apply ka_complete_weq. now rewrite <-2G_hat, H.
apply w_weq in H.
rewrite 2wvo_uo in H.
apply erase_faithful_weq in H.
2: now rewrite 2o_level.
2: reflexivity.
apply o_inj in H.
now rewrite 2teval_hat in H.
Qed.

Corollary kat_dec n m: e f: gregex n m, {e f} + {~(e f)}.
Proof. intros. eapply sumbool_iff. symmetry. apply kat_reduces_to_ka. apply ka_weq_dec. Qed.

End KatCompleteness.