Goal:
  
  A : Type
  P : list A → list A → A → A → Type
  n : nat
  d : A
  Γ0 : list A
  a : A
  Γ' : list A
  d' : A
  IHn : ∀ (Γ Γ' : list A) (d' : A),
          All2_fold P Γ Γ'
          → nth_error Γ' n = Some d'
            → ∑ d : A,
                nth_error Γ n = Some d
                × All2_fold P (skipn (S n) Γ) (skipn (S n) Γ')
                  × P (skipn (S n) Γ) (skipn (S n) Γ') d d'
  Hrel : All2_fold P Γ0 Γ'
  p : P Γ0 Γ' d a
  H : nth_error Γ' n = Some d'
  x : A
  a0 : nth_error Γ0 n = Some x
  a1 : All2_fold P (skipn (S n) Γ0) (skipn (S n) Γ')
  b0 : P (skipn (S n) Γ0) (skipn (S n) Γ') x d'
  ============================
   (P (skipn (S (S n)) (d :: Γ0)) (skipn (S (S n)) (a :: Γ'))
      ?d@{p0:=(a0, (a1, b0))} d')


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