Goal:
  
  A : Type
  B : Type
  P : B → A → A → Prop
  h : ∀ b : B, Transitive (P b)
  l'' : list A
  x : B
  y : A
  z : A
  l : list B
  l' : list A
  l''0 : list A
  H : P x y z
  hl : Forall3 P l l' l''0
  IHhl : ∀ l'' : list A, Forall3 P l l''0 l'' → Forall3 P l l' l''
  z0 : A
  l''1 : list A
  H0 : P x z z0
  H1 : Forall3 P l l''0 l''1
  ============================
   (P x y ?y)


DEBUG:Going to execute:
repeat
            (match reverse goal with
             | id:?X1
               |- _ =>
                   <Coq.Init.Tauto.is_conj> flags X1; elim id; do 2 intro;
                    clear id
             | id:_ ↔ _ |- _ => elim id; do 2 intro; clear id
             | id:¬ _ |- _ => red in id
             | id:?X1
               |- _ =>
                   <Coq.Init.Tauto.is_disj> flags X1; elim id; intro; clear
                    id
             | _ =>
                 <Coq.Init.Tauto.find_cut>
                  ltac:(fun id0 id1 X2 =>
                          assert X2; [ exact (id0 id1) | clear id0 ])
             | id:?X1 → ?X2
               |- _ =>
                   <Coq.Init.Tauto.is_unit_or_eq> flags X1; Ltac.cut X2;
                    [ intro; clear id
                    | Ltac.cut X1; [ exact id | constructor 1; fail ] ]
             | id:?X1 → ?X2
               |- _ =>
                   <Coq.Init.Tauto.flatten_contravariant_conj> flags X1 X2 id
             | id:?X1 ↔ ?X2 → ?X3
               |- _ =>
                   assert ((X1 → X2) → (X2 → X1) → X3) by
                    (do 2 intro; apply id; Ltac.split; Ltac.assumption);
                    clear id
             | id:?X1 → ?X2
               |- _ =>
                   <Coq.Init.Tauto.flatten_contravariant_disj> flags X1 X2 id
             | |- ?X1 => <Coq.Init.Tauto.is_conj> flags X1; Ltac.split
             | |- _ ↔ _ => Ltac.split
             | |- ¬ _ => red
             end; <Coq.Init.Tauto.not_dep_intros>)
Level 0: Backtrace:

No matching clauses for match.