Goal:
  
  A : Type
  P : list A → list A → A → A → Type
  n : nat
  a : A
  Γ : list A
  d' : A
  Γ'0 : 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 Γ Γ'0
  p : P Γ Γ'0 a d'
  H : nth_error Γ n = Some d
  x : A
  a0 : nth_error Γ'0 n = Some x
  b : All2_fold P (skipn (S n) Γ) (skipn (S n) Γ'0)
      × P (skipn (S n) Γ) (skipn (S n) Γ'0) d x
  ============================
   (nth_error Γ'0 n = Some ?d'@{p0:=(a0, b)}
    × All2_fold P (skipn (S (S n)) (a :: Γ)) (skipn (S (S n)) (d' :: Γ'0))
      × P (skipn (S (S n)) (a :: Γ)) (skipn (S (S n)) (d' :: Γ'0)) d
          ?d'@{p0:=(a0, b)})


DEBUG:Going to execute:
<Coq.Init.Tauto.is_conj> flags X1; elim id; do 2 intro; clear id
Level 2: evaluation returns
<coq-core.plugins.tauto::is_conj@0>
where
tauto_flags := 
of type
tauto_flags
X1 := nat
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 1: Backtrace:

Tactic failure.
Level 1: Backtrace:

Tactic failure.