Goal:
  
  A : Type
  B : Type
  P : B → B → Type
  Q : A → B → Type
  R : A → B → Type
  x : A
  l : list A
  p : ∀ y z : B, Q x y → R x z → ∑ v : B, P y v × P z v
  H : All (λ x : A, ∀ y z : B, Q x y → R x z → ∑ v : B, P y v × P z v) l
  IHAll : ∀ l' l'' : list B,
            All2 Q l l'
            → All2 R l l''
              → ∑ l''' : list B, All2 P l' l''' × All2 P l'' l'''
  y : B
  l'0 : list B
  H' : All2 Q (x :: l) (y :: l'0)
  y0 : B
  l'1 : list B
  H'' : All2 R (x :: l) (y0 :: l'1)
  X : Q x y
  X0 : All2 Q l l'0
  X1 : R x y0
  X2 : All2 R l l'1
  x0 : list B
  a : All2 P l'0 x0
  a0 : All2 P l'1 x0
  x1 : B
  p0 : P y x1
  p1 : P y0 x1
  ============================
   (All2 P l'0 x0)


DEBUG:Going to execute:
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 2: evaluation returns
<coq-core.plugins.tauto::is_conj@0>
where
tauto_flags := 
of type
tauto_flags
X1 := P y x1
of type
constr of type tacvalue

Level 2: Backtrace:

Tactic failure.
Level 2: Backtrace:

Tactic failure.
Level 1: Backtrace:

Tactic failure.
Level 1: Backtrace:

Tactic failure.
Level 0: Backtrace:

No matching clauses for match.
Level 0: Backtrace:

No matching clauses for match.