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
  p0 : nth_error Γ0 n = Some x
       × All2_fold P (skipn (S n) Γ0) (skipn (S n) Γ')
         × P (skipn (S n) Γ0) (skipn (S n) Γ') x d'
  ============================
   (nth_error Γ0 n = Some ?d
    × All2_fold P (skipn (S (S n)) (d :: Γ0)) (skipn (S (S n)) (a :: Γ'))
      × P (skipn (S (S n)) (d :: Γ0)) (skipn (S (S n)) (a :: Γ')) ?d d')


DEBUG:Going to execute:
fun flags t_reduce t_solver =>
             let rec t_tauto_intuit :=
              <Coq.Init.Tauto.simplif> flags;
               <Coq.Init.Tauto.axioms> flags ||
                 match reverse goal with
                 | id:(?X1 → ?X2) → ?X3
                   |- _ =>
                       Ltac.cut X3;
                        [ intro; clear id; t_tauto_intuit
                        | Ltac.cut (X1 → X2);
                           [ exact
                           id
                           | generalize (λ y : X2, id (λ x : X1, y)); intro;
                              clear id; (solve [ t_tauto_intuit ]) ] ]
                 | id:¬ ?X1 → ?X3
                   |- _ =>
                       Ltac.cut X3;
                        [ intro; clear id; t_tauto_intuit
                        | Ltac.cut (¬ X1);
                           [ exact
                           id
                           | clear id; intro; (solve [ t_tauto_intuit ]) ] ]
                 | |- ?X1 =>
                       <Coq.Init.Tauto.is_disj> flags X1; (solve
                        [ Ltac.left; t_tauto_intuit
                        | Ltac.right; t_tauto_intuit ])
                 end ||
                   match goal with
                   | |- _ → _ => intro; t_tauto_intuit
                   | |- _ => t_reduce; t_solver
                   end || t_solver
             in
             t_tauto_intuit