Require Import RelationClasses.

From mathcomp Require Import all_ssreflect.
Require Import edone preliminaries digraph sgraph treewidth.
Require Import set_tac.

Set Implicit Arguments.
Unset Strict Implicit.
Unset Printing Implicit Defensive.

Local Open Scope quotient_scope.
Set Bullet Behavior "Strict Subproofs".

Definition minor_map (G H : sgraph) (phi : G -> option H) :=
  [/\ (forall y : H, exists x : G, phi x = Some y),
     (forall y : H, connected (phi @^-1 Some y)) &
     (forall x y : H, x -- y -> exists x0 y0 : G,
      [/\ x0 \in phi @^-1 Some x, y0 \in phi @^-1 Some y & x0 -- y0])].

Definition minor_rmap (G H : sgraph) (phi : H -> {set G}) :=
  [/\ (forall x : H, phi x != set0),
     (forall x : H, connected (phi x)),
     (forall x y : H, x != y -> [disjoint phi x & phi y]) &
     (forall x y : H, x -- y -> neighbor (phi x) (phi y))].

Lemma minor_map_rmap (G H : sgraph) (phi : H -> {set G}) :
  minor_rmap phi -> minor_map (fun x : G => [pick x0 : H | x \in phi x0]).
Proof.
  set phi' := (fun x => _).
  case => P1 P2 P3 P4.
  have phiP x x0 : x0 \in phi x = (phi' x0 == Some x).
  { rewrite /phi'. case: pickP => [x' Hx'|]; last by move->.
    rewrite Some_eqE. apply/idP/eqP => [|<-//].
    apply: contraTeq => /P3 D. by rewrite (disjointFr D Hx'). }
  split.
  - move => y. case/set0Pn : (P1 y) => y0. rewrite phiP => /eqP <-. by exists y0.
  - move => y x0 y0. rewrite !inE -!phiP => H1 H2. move: (P2 y _ _ H1 H2).
    apply: connect_mono => u v. by rewrite /= -!mem_preim !phiP.
  - move => x y /P4/neighborP [x0] [y0] [*]. exists x0;exists y0. by rewrite -!mem_preim -!phiP.
Qed.

Lemma minor_rmap_map (G H : sgraph) (phi : G -> option H) :
  minor_map phi -> minor_rmap (fun x => [set y | phi y == Some x]).
Proof.
  set phi' := fun _ => _.
  case => P1 P2 P3.
  split.
  - move => x. apply/set0Pn. case: (P1 x) => x0 H0. exists x0. by rewrite !inE H0.
  - move => x u v Hu Hv. move: (P2 x _ _ Hu Hv).
    apply: connect_mono => a b. by rewrite /= !inE.
  - move => x y D. rewrite disjoint_exists.
    apply: contraNN D => /exists_inP [x0]. by rewrite -Some_eqE !inE => /eqP<-/eqP<-.
  - move => x y /P3 [x0] [y0] [*]. apply/neighborP. exists x0;exists y0. by rewrite !inE !mem_preim.
Qed.

Lemma rmap_add_edge_sym (G H : sgraph) (s1 s2 : G) (phi : H -> {set G}) :
  @minor_rmap (add_edge s1 s2) H phi -> @minor_rmap (add_edge s2 s1) H phi.
Proof.
  case => [P1 P2 P3 P4]; split => //.
  - move => x. exact/add_edge_connected_sym.
  - move => x y /P4/neighborP => [[x'] [y'] [A B C]].
    apply/neighborP; exists x'; exists y'. by rewrite add_edgeC.
Qed.

Definition minor (G H : sgraph) : Prop := exists phi : G -> option H, minor_map phi.

Fact minor_of_map (G H : sgraph) (phi : G -> option H):
  minor_map phi -> minor G H.
Proof. case => *. by exists phi. Qed.

Fact minor_of_rmap (G H : sgraph) (phi : H -> {set G}):
  minor_rmap phi -> minor G H.
Proof. move/minor_map_rmap. exact: minor_of_map. Qed.

Lemma minorRE G H : minor G H -> exists phi : H -> {set G}, minor_rmap phi.
Proof. case => phi /minor_rmap_map D. eexists. exact: D. Qed.

Lemma mem_bigcup (T1 T2 : finType) (F : T1 -> {set T2}) (P : pred T1) z y :
  P y -> z \in F y -> z \in \bigcup_(x | P x) F x.
Proof. move => Py zF. by apply/bigcupP; exists y; rewrite ?yA. Qed.
Arguments mem_bigcup [T1 T2 F P z] y _ _.

Lemma minor_rmap_comp (G H K : sgraph) (f : H -> {set G}) (g : K -> {set H}) :
  minor_rmap f -> minor_rmap g -> minor_rmap (fun x => \bigcup_(y in g x) f y).
Proof.
  move => [f1 f2 f3 f4] [g1 g2 g3 g4]. split => [x|x|x1 x2|x1 x2].
  - case/set0Pn: (g1 x) => y Gy; case/set0Pn: (f1 y) => z Fz.
    by apply/set0Pn; exists z; apply/bigcupP; exists y.
  - move => z1 z2 /bigcupP [y1 y1_g z1_f] /bigcupP [y2 y2_g z2_f].
    have/connectP [p] := (g2 _ _ _ y1_g y2_g).
    elim: p z1 y1 y1_g z1_f => /= [|y1' p IHp] z1 y1 y1_g z1_f.
    + move => _ ?; subst.
      apply: connect_restrict_mono; [exact: bigcup_sup y1_g|exact: f2].
    + rewrite !in_simpl -andbA => /and3P [/andP [H1 H2] H3 H4 H5].
      case/neighborP: (f4 _ _ H3) => a [b] [a_fy1 b_fy1' ab].
      apply: connect_trans (IHp _ _ H2 b_fy1' H4 H5).
      apply: (@connect_trans _ _ a).
      * apply: connect_restrict_mono. apply: bigcup_sup y1_g. exact: f2.
      * apply: connect1 => /=.
        by rewrite !in_simpl ab andbT (mem_bigcup y1) ?(mem_bigcup y1').
  - move/g3 => Dx. apply/disjointP => z.
    case/bigcupP => y1 y1_g z_fy1; case/bigcupP => y2 y2_g z_fy2.
    suff: y1 != y2 by move/f3/disjointP/(_ z); apply.
    apply: contraTneq Dx => ?;subst. exact: disjointNI y2_g.
  - move/g4/neighborP => [y1] [y2] [Y1 Y2 /f4 /neighborP [z1] [z2] [? ? e]].
    by apply/neighborP; exists z1; exists z2; rewrite (mem_bigcup y1) ?(mem_bigcup y2).
Qed.

Lemma minor_map_comp (G H K : sgraph) (f : G -> option H) (g : H -> option K) :
  minor_map f -> minor_map g -> minor_map (obind g \o f).
Proof.
  move=> [f1 f2 f3] [g1 g2 g3]. rewrite /comp; split.
  - move => y. case: (g1 y) => y'. case: (f1 y') => x E1 ?.
    exists x. by rewrite E1.
  - move => z x y. rewrite !inE.
    case Ef : (f x) => [fx|] //= gfx. case Eg : (f y) => [fy|] //= gfy.
    move: (g2 z fx fy). rewrite !inE. case/(_ _ _)/Wrap => // /connectP => [[p]].
    elim: p x fx Ef gfx => /= [|a p IH] x fx Ef gfx.
    + move => _ ?. subst fy.
      move: (f2 fx x y). rewrite !inE Ef Eg. case/(_ _ _)/Wrap => //.
      apply: connect_mono => a b /=. rewrite !inE -andbA.
      case/and3P => /eqP-> /eqP-> -> /=. by rewrite (eqP gfx) !eqxx.
    + rewrite !inE -!andbA => /and4P [H1 H2 H3 H4] H5.
      case: (f1 a) => x' Hx'. apply: (connect_trans (y := x')); last exact: IH H5.
      move/f3 : (H3) => [x0] [y0] [X1 X2 X3].
      apply: (connect_trans (y := x0)); last apply: (connect_trans (y := y0)).
      * move: (f2 fx x x0). rewrite !inE ?Ef ?eqxx in X1 *. case/(_ _ _)/Wrap => //.
        apply: connect_mono => u v /=. rewrite !inE -andbA.
        case/and3P => /eqP-> /eqP-> -> /=. by rewrite H1.
      * apply: connect1. rewrite /= !inE ?X3 ?andbT in X1 X2 *.
        by rewrite (eqP X1) (eqP X2) /= (eqP gfx) eqxx.
      * move: (f2 a y0 x' X2). case/(_ _)/Wrap. by rewrite !inE Hx'.
        apply: connect_mono => u v /=. rewrite !inE -andbA.
        case/and3P => /eqP-> /eqP-> -> /=. by rewrite H2.
  - move => x y /g3 [x'] [y'] [Hx' Hy'] /f3 [x0] [y0] [Hx0 Hy0 ?].
    exists x0. exists y0. rewrite !inE in Hx' Hy' Hx0 Hy0 *.
    split => //; reflect_eq; by rewrite (Hx0,Hy0) /= (Hx',Hy').
Qed.

Lemma minor_trans : Transitive minor.
Proof.
  move => G H I /minorRE [f mm_f] /minorRE [g mm_g].
  apply: minor_of_rmap. exact: minor_rmap_comp mm_f mm_g.
Qed.

Definition total_minor_map (G H : sgraph) (phi : G -> H) :=
  [/\ (forall y : H, exists x, phi x = y),
     (forall y : H, connected (phi @^-1 y)) &
     (forall x y : H, x -- y ->
     exists x0 y0, [/\ x0 \in phi @^-1 x, y0 \in phi @^-1 y & x0 -- y0])].

Definition strict_minor (G H : sgraph) : Prop :=
  exists phi : G -> H, total_minor_map phi.

Lemma map_of_total (G H : sgraph) (phi : G -> H) :
  total_minor_map phi -> minor_map (Some \o phi).
Proof. case => A B C. split => // y. case: (A y) => x <-. by exists x. Qed.

Lemma strict_is_minor (G H : sgraph) : strict_minor G H -> minor G H.
Proof. move => [phi A]. exists (Some \o phi). exact: map_of_total. Qed.

Lemma sub_minor (S G : sgraph) : subgraph S G -> minor G S.
Proof.
  move => [h inj_h hom_h].
  pose phi x := if @idP (x \in codom h) is ReflectT p then Some (iinv p) else None.
  exists phi; split.
  - move => y. exists (h y). rewrite /phi.
    case: {-}_ / idP => [p|]; by rewrite ?iinv_f ?codom_f.
  - move => y x0 y0. rewrite !inE {1 2}/phi.
    case: {-}_ / idP => // p /eqP[E1].
    case: {-}_ / idP => // q /eqP[E2].
    suff -> : (x0 = y0) by exact: connect0.
    by rewrite -(f_iinv p) -(f_iinv q) E1 E2.
  - move => x y A. move/hom_h : (A) => B.
    exists (h x). exists (h y). rewrite !inE /phi B.
    + by do 2 case: {-}_ / idP => [?|]; rewrite ?codom_f ?iinv_f ?eqxx //.
    + apply: contraTneq A => /inj_h ->. by rewrite sgP.
Qed.

Lemma iso_strict_minor (G H : sgraph) : diso G H -> strict_minor H G.
Proof.
  (* TODO: update proof to abstract against concrete implem of diso *)
  move=> [[h g hgK ghK] /= hH gH].
  have in_preim_g x y : (y \in g @^-1 x) = (y == h x).
    rewrite -mem_preim; exact: can2_eq.
  exists g; split.
  + by move=> y; exists (h y); rewrite hgK.
  + move=> y x1 x2. rewrite !in_preim_g => /eqP-> /eqP->. exact: connect0.
  + move=> x y xy. exists (h x); exists (h y). rewrite !in_preim_g.
    split=> //. exact: hH.
Qed.

Lemma induced_minor (G : sgraph) (S : {set G}) : minor G (induced S).
Proof. apply: sub_minor. exact: induced_sub. Qed.

Definition edge_surjective (G1 G2 : sgraph) (h : G1 -> G2) :=
  forall x y : G2 , x -- y -> exists x0 y0, [/\ h x0 = x, h y0 = y & x0 -- y0].

(* The following should hold but does not fit the use case for minors *)
Lemma rename_sdecomp (T : forest) (G H : sgraph) D (dec_D : sdecomp T G D) (h :G -> H) :
  hom_s h -> surjective h -> edge_surjective h ->
  (forall x y, h x = h y -> exists t, (x \in D t) && (y \in D t)) ->
  @sdecomp T _ (rename D h).
Abort.

Lemma width_minor (G H : sgraph) (T : forest) (B : T -> {set G}) :
  sdecomp T G B -> minor G H -> exists B', @sdecomp T H B' /\ width B' <= width B.
Proof.
  move => decT [phi [p1 p2 p3]].
  pose B' t := [set x : H | [exists (x0 | x0 \in B t), phi x0 == Some x]].
  exists B'. split.
  - split.
    + move => y. case: (p1 y) => x /eqP Hx.
      case: (sbag_cover decT x) => t Ht.
      exists t. apply/pimsetP. by exists x.
    + move => x y xy. move/p3: xy => [x0] [y0]. rewrite !inE => [[H1 H2 H3]].
      case: (sbag_edge decT H3) => t /andP [T1 T2]. exists t.
      apply/andP; split; apply/pimsetP; by [exists x0|exists y0].
    + have conn_pre1 t1 t2 x x0 :
        phi x0 == Some x -> x0 \in B t1 -> x0 \in B t2 ->
        connect (restrict [pred t | x \in B' t] sedge) t1 t2.
      { move => H1 H2 H3. move: (sbag_conn decT H2 H3).
        apply: connect_mono => u v /=. rewrite !in_simpl -!andbA => /and3P [? ? ?].
        apply/and3P; split => //; apply/pimsetP; eexists; eauto. }
      move => x t1 t2 /pimsetP [x0 X1 X2] /pimsetP [y0 Y1 Y2].
      move: (p2 x x0 y0). rewrite !inE. case/(_ _ _)/Wrap => // /connectP [p].
      elim: p t1 x0 X1 X2 => /= [|z0 p IH] t1 x0 X1 X2.
      * move => _ E. subst x0. exact: conn_pre1 X1 Y1.
      * rewrite -!andbA => /and3P [H1 H2 /andP [H3 H4] H5].
        case: (sbag_edge decT H3) => t /andP [T1 T2].
        apply: (connect_trans (y := t)).
        -- move => {p IH H4 H5 y0 Y1 Y2 X2}. rewrite !inE in H1 H2.
           exact: conn_pre1 X1 T1.
        -- apply: IH H4 H5 => //. by rewrite inE in H2.
  - apply: max_mono => t. exact: pimset_card.
Qed.

Lemma minor_of_clique (G : sgraph) (S : {set G}) n :
  n <= #|S| -> clique S -> minor G 'K_n.
Proof.
  case/card_gtnP => s [uniq_s /eqP size_s sub_s clique_S].
  pose t := Tuple size_s.
  pose phi (i : 'K_n) := [set tnth t i].
  suff H: minor_rmap phi by apply (minor_of_map (minor_map_rmap H)).
  split.
  - move => i. apply/set0Pn; exists (tnth t i). by rewrite !inE.
  - move => i. exact: connected1.
  - move => i j iNj. rewrite disjoints1. apply: contraNN iNj.
    by rewrite inE tnth_uniq.
  - move => i j /= ?. apply/neighborP. exists (tnth t i); exists (tnth t j).
    rewrite !inE !tnth_uniq ?eqxx //.
    rewrite clique_S // ?tnth_uniq // ?sub_s //; exact: mem_tnth.
Qed.

Lemma Kn_clique n : clique [set: 'K_n].
Proof. by []. Qed.

Definition K4_free (G : sgraph) := ~ minor G K4.

Lemma minor_K4_free (G H : sgraph) :
  minor G H -> K4_free G -> K4_free H.
Proof. move => M F C. apply: F. exact: minor_trans C. Qed.

Lemma subgraph_K4_free (G H : sgraph) :
  subgraph H G -> K4_free G -> K4_free H.
Proof. move/sub_minor. exact: minor_K4_free. Qed.

Lemma iso_K4_free (G H : sgraph) :
  diso G H -> K4_free H -> K4_free G.
Proof. move => iso_GH. apply: subgraph_K4_free. exact: iso_subgraph. Qed.

Lemma treewidth_K_free (G : sgraph) (T : forest) (B : T -> {set G}) m :
  sdecomp T G B -> width B <= m -> ~ minor G 'K_m.+1.
Proof.
  move => decT wT M. case: (width_minor decT M) => B' [B1 B2].
  suff: m < m by rewrite ltnn.
  apply: leq_trans wT. apply: leq_trans B2. apply: (Km_width B1).
Qed.

Lemma TW2_K4_free (G : sgraph) (T : forest) (B : T -> {set G}) :
  sdecomp T G B -> width B <= 3 -> K4_free G.
Proof. exact: treewidth_K_free. Qed.

Lemma small_K_free m (G : sgraph): #|G| <= m -> ~ minor G 'K_m.+1.
Proof.
  move => H. case: (decomp_small H) => T [D] [decD wD].
  exact: treewidth_K_free decD wD.
Qed.

(* TODO: theory for induced [set~ : None : add_node] *)
Lemma minor_induced_add_node (G : sgraph) (N : {set G}) : @minor_map (induced [set~ None : add_node G N]) G val.
Proof.
  have inNoneD (a : G) : Some a \in [set~ None] by rewrite !inE. split.
  + move=> y. by exists (Sub (Some y) (inNoneD y)).
  + move=> y x1 x2. rewrite -!mem_preim =>/eqP<- /eqP/val_inj->. exact: connect0.
  + move=> x y xy. exists (Sub (Some x) (inNoneD x)).
    exists (Sub (Some y) (inNoneD y)). by split; rewrite -?mem_preim.
Qed.

Lemma add_node_minor (G G' : sgraph) (U : {set G}) (U' : {set G'}) (phi : G -> G') :
  (forall y, y \in U' -> exists2 x, x \in U & phi x = y) ->
  total_minor_map phi ->
  minor (add_node G U) (add_node G' U').
Proof.
  move => H [M1 M2 M3].
  apply: strict_is_minor. exists (omap phi). split.
  - case => [y|]; last by exists None. case: (M1 y) => x E.
    exists (Some x). by rewrite /= E.
  - move => [y|].
    + rewrite preim_omap_Some. exact: connected_add_node.
    + rewrite preim_omap_None. exact: connected1.
  - move => [x|] [y|] //=.
    + move/M3 => [x0] [y0] [H1 H2 H3]. exists (Some x0); exists (Some y0).
      by rewrite !preim_omap_Some !mem_imset.
    + move/H => [x0] H1 H2. exists (Some x0); exists None.
      rewrite !preim_omap_Some !preim_omap_None !inE !eqxx !mem_imset //.
      by rewrite -mem_preim H2.
    + move/H => [y0] H1 H2. exists None; exists (Some y0).
      rewrite !preim_omap_Some !preim_omap_None !inE !eqxx !mem_imset //.
      by rewrite -mem_preim H2.
Qed.

Lemma minor_with (H G': sgraph) (S : {set H}) (i : H) (N : {set G'})
  (phi : (sgraph.induced S) -> option G') :
  i \notin S ->
  (forall y, y \in N -> exists2 x , x \in phi @^-1 (Some y) & val x -- i) ->
  @minor_map (sgraph.induced S) G' phi ->
  minor H (add_node G' N).
Proof.
  move => Hi Hphi mm_phi.
  pose psi (u:H) : option (add_node G' N) :=
    match @idP (u \in S) with
    | ReflectT p => obind (fun x => Some (Some x)) (phi (Sub u p))
    | ReflectF _ => if u == i then Some None else None
    end.
  (* NOTE: use (* case: {-}_ / idP *) to analyze psi *)
  have psi_G' (a : G') : psi @^-1 (Some (Some a)) = val @: (phi @^-1 (Some a)).
  { apply/setP => x. rewrite !inE. apply/eqP/imsetP.
    + rewrite /psi. case: {-}_ / idP => p; last by case: ifP.
      case E : (phi _) => [b|//] /= [<-]. exists (Sub x p) => //. by rewrite !inE E.
    + move => [[/= b Hb] Pb] ->. rewrite /psi. case: {-}_ / idP => //= Hb'.
      rewrite !inE (bool_irrelevance Hb Hb') in Pb. by rewrite (eqP Pb). }
  have psi_None : psi @^-1 (Some None) = [set i].
  { apply/setP => z. rewrite !inE /psi.
    case: {-}_ / idP => [p|_]; last by case: ifP.
    have Hz : z != i. { apply: contraNN Hi. by move/eqP <-. }
    case: (phi _) => [b|]; by rewrite (negbTE Hz). }
  case: mm_phi => M1 M2 M3. exists psi;split.
  - case.
    + move => a. case: (M1 a) => x E. exists (val x). apply/eqP.
      rewrite mem_preim psi_G' mem_imset //. by rewrite !inE E.
    + exists i. rewrite /psi. move: Hi.
      case: {-}_ / idP => [? ?|_ _]; by [contrab|rewrite eqxx].
  - case.
    + move => y. move: (M2 y). rewrite psi_G'. exact: connected_in_subgraph.
    + rewrite psi_None. exact: connected1.
  - move => [a|] [b|]; last by rewrite sg_irrefl.
    + move => /= /M3 [x0] [y0] [? ? ?].
      exists (val x0). exists (val y0). by rewrite !psi_G' !mem_imset.
    + move => /= /Hphi [x0] ? ?. exists (val x0); exists i. by rewrite psi_None set11 !psi_G' !mem_imset.
    + move => /= /Hphi [x0] ? ?. exists i;exists (val x0). by rewrite sg_sym psi_None set11 !psi_G' !mem_imset.
Qed.

(* TODO: use this whenever explicitly exhibiting a minor map *)
Lemma minor_rmapI (G H : sgraph) (phi : H -> {set G}) (f : H -> nat) :
  injective f ->
  (forall x : H, phi x != set0) ->
  (forall x : H, connected (phi x)) ->
  (forall x y : H, f x < f y -> [disjoint phi x & phi y]) ->
  (forall x y : H, f x < f y -> x -- y -> neighbor (phi x) (phi y)) ->
  minor_rmap phi.
Proof.
  move => inj_f M1 M2 M3 M4. split => // x y xy.
  - wlog: x y xy / f x < f y; last exact: M3.
    move => W. case: (ltngtP (f x) (f y)); first exact: W.
    + rewrite disjoint_sym eq_sym in xy *. exact: W.
    + move/inj_f => E. by rewrite E eqxx in xy.
  - wlog: x y xy / f x < f y; last by move => Hf; exact: M4 Hf xy.
    move => W. case: (ltngtP (f x) (f y)); first exact: W.
    + rewrite neighborC sgP in xy *. exact: W.
    + move/inj_f => E. by rewrite E sgP in xy.
Qed.

Lemma non_forerst_K3 (G : sgraph) : ~ is_forest [set: G] -> minor G 'K_3.
Proof.
  move/is_forestP/is_forestPn => [x0] [y0] [p0] [q0] [_ _ pDq].
  have [x [y] [p1] [p2] [p12_disj p1_ne]] := disjoint_part (valP p0) (valP q0) pDq.
  clear x0 y0 p0 q0 pDq.
  pose phi (i : 'K_3) : {set G} :=
    match i with
    | Ordinal 0 _ => [set x]
    | Ordinal 1 _ => interior p1
    | Ordinal 2 _ => y |: interior p2
    | Ordinal _ _ => set0
    end.
  suff: minor_rmap phi by apply: minor_of_rmap.
  have xDy : x != y.
  { apply: contra_neq p1_ne => ?; subst y.
    by rewrite /path_of_ipath (irredxx (valP p1)) interior_idp. }
  apply: minor_rmapI; first exact: ord_inj.
  - case => [[|[|[|i]]] Hi] //=; [exact: set10 | exact: setU1_neq].
  - case => [[|[|[|i]]] Hi] //=.
    + exact: connected1.
    + exact: connected_interior.
    + exact: connected_interiorR.
  - case => [[|[|[|i]]] Hi]; case => [[|[|[|j]]] Hj] //= _.
    + by rewrite disjoints1 !inE eqxx.
    + by rewrite disjoints1 !inE eqxx (negbTE xDy).
    + rewrite disjoint_sym disjointsU // ?disjoints1 1?disjoint_sym //.
      by rewrite !inE eqxx.
  - case => [[|[|[|i]]] Hi]; case => [[|[|[|j]]] Hj] //= _ _.
    + apply: path_neighborL => //. by rewrite inE.
    + exact: neighbor_interiorL.
    + apply: neighborUl. rewrite neighborC.
      apply: path_neighborR => //. by rewrite inE.
Qed.

Theorem K3_free_forest G : ~ minor G 'K_3 <-> is_forest [set: G].
Proof.
  split.
  - rewrite (rwP (is_forestP _)). apply: contra_notT. move/is_forestP.
    exact: non_forerst_K3.
  - case/forest_TW1 => T [B []]. exact: treewidth_K_free.
Qed.

Theorem TW1_forest G : (exists T B, sdecomp T G B /\ width B <= 2) <-> is_forest [set: G].
Proof.
  split => [[T] [B] [decB wB]|]; last exact: forest_TW1.
  apply/K3_free_forest. exact: treewidth_K_free decB wB.
Qed.