Goal:
  
  A : Type
  P : list A → list A → A → A → Type
  n : nat
  a : A
  Γ : list 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'
                × (let Γs := skipn (S n) Γ in
                   let Γs' := skipn (S n) Γ' in
                   All2_fold P Γs Γs' × P Γs Γs' d d')
  Hrel : All2_fold P (a :: Γ) Γ'
  H : nth_error (a :: Γ) (S n) = Some d
  ============================
   (let H := CoreTactics.block in
    ∑ d' : A,
      nth_error Γ' (S n) = Some d'
      × (let Γs := skipn (S (S n)) (a :: Γ) in
         let Γs' := skipn (S (S n)) Γ' in All2_fold P Γs Γs' × P Γs Γs' d d'))


DEBUG:Going to execute:
idtac
Level 5: id'
 has value 
H


Goals:
  
  A : Type
  P : list A → list A → A → A → Type
  Γ' : list A
  d : A
  Hrel : All2_fold P [] Γ'
  H : nth_error [] 0 = Some d
  ============================
   (let H := CoreTactics.block in
    ∑ d' : A,
      nth_error Γ' 0 = Some d'
      × (let Γs := skipn 1 [] in
         let Γs' := skipn 1 Γ' in All2_fold P Γs Γs' × P Γs Γs' d d'))

  
  A : Type
  P : list A → list A → A → A → Type
  a : A
  Γ : list A
  Γ' : list A
  d : A
  Hrel : All2_fold P (a :: Γ) Γ'
  H : nth_error (a :: Γ) 0 = Some d
  ============================
   (let H := CoreTactics.block in
    ∑ d' : A,
      nth_error Γ' 0 = Some d'
      × (let Γs := skipn 1 (a :: Γ) in
         let Γs' := skipn 1 Γ' in All2_fold P Γs Γs' × P Γs Γs' d d'))

  
  A : Type
  P : list A → list A → A → A → Type
  n : nat
  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'
                × (let Γs := skipn (S n) Γ in
                   let Γs' := skipn (S n) Γ' in
                   All2_fold P Γs Γs' × P Γs Γs' d d')
  Hrel : All2_fold P [] Γ'
  H : nth_error [] (S n) = Some d
  ============================
   (let H := CoreTactics.block in
    ∑ d' : A,
      nth_error Γ' (S n) = Some d'
      × (let Γs := skipn (S (S n)) [] in
         let Γs' := skipn (S (S n)) Γ' in All2_fold P Γs Γs' × P Γs Γs' d d'))

  
  A : Type
  P : list A → list A → A → A → Type
  n : nat
  a : A
  Γ : list 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'
                × (let Γs := skipn (S n) Γ in
                   let Γs' := skipn (S n) Γ' in
                   All2_fold P Γs Γs' × P Γs Γs' d d')
  Hrel : All2_fold P (a :: Γ) Γ'
  H : nth_error (a :: Γ) (S n) = Some d
  ============================
   (let H := CoreTactics.block in
    ∑ d' : A,
      nth_error Γ' (S n) = Some d'
      × (let Γs := skipn (S (S n)) (a :: Γ) in
         let Γs' := skipn (S (S n)) Γ' in All2_fold P Γs Γs' × P Γs Γs' d d'))


DEBUG:Going to execute:
CoreTactics.block_goal
Level 4: evaluation returns
idtac of type tacvalue

Level 2: evaluation returns
idtac of type tacvalue

Level 2: evaluation returns
idtac of type tacvalue

Level 2: evaluation returns
idtac of type tacvalue

Level 2: evaluation returns
idtac of type tacvalue

Level 1: evaluation returns
idtac of type tacvalue

Level 1: evaluation returns
idtac of type tacvalue

Level 1: evaluation returns
idtac of type tacvalue

Level 1: evaluation returns
idtac of type tacvalue


Goals:
  
  A : Type
  P : list A → list A → A → A → Type
  Γ' : list A
  d : A
  Hrel : All2_fold P [] Γ'
  H : nth_error [] 0 = Some d
  ============================
   (let H := CoreTactics.block in
    ∑ d' : A,
      nth_error Γ' 0 = Some d'
      × (let Γs := skipn 1 [] in
         let Γs' := skipn 1 Γ' in All2_fold P Γs Γs' × P Γs Γs' d d'))

  
  A : Type
  P : list A → list A → A → A → Type
  a : A
  Γ : list A
  Γ' : list A
  d : A
  Hrel : All2_fold P (a :: Γ) Γ'
  H : nth_error (a :: Γ) 0 = Some d
  ============================
   (let H := CoreTactics.block in
    ∑ d' : A,
      nth_error Γ' 0 = Some d'
      × (let Γs := skipn 1 (a :: Γ) in
         let Γs' := skipn 1 Γ' in All2_fold P Γs Γs' × P Γs Γs' d d'))

  
  A : Type
  P : list A → list A → A → A → Type
  n : nat
  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'
                × (let Γs := skipn (S n) Γ in
                   let Γs' := skipn (S n) Γ' in
                   All2_fold P Γs Γs' × P Γs Γs' d d')
  Hrel : All2_fold P [] Γ'
  H : nth_error [] (S n) = Some d
  ============================
   (let H := CoreTactics.block in
    ∑ d' : A,
      nth_error Γ' (S n) = Some d'
      × (let Γs := skipn (S (S n)) [] in
         let Γs' := skipn (S (S n)) Γ' in All2_fold P Γs Γs' × P Γs Γs' d d'))

  
  A : Type
  P : list A → list A → A → A → Type
  n : nat
  a : A
  Γ : list 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'
                × (let Γs := skipn (S n) Γ in
                   let Γs' := skipn (S n) Γ' in
                   All2_fold P Γs Γs' × P Γs Γs' d d')
  Hrel : All2_fold P (a :: Γ) Γ'
  H : nth_error (a :: Γ) (S n) = Some d
  ============================
   (let H := CoreTactics.block in
    ∑ d' : A,
      nth_error Γ' (S n) = Some d'
      × (let Γs := skipn (S (S n)) (a :: Γ) in
         let Γs' := skipn (S (S n)) Γ' in All2_fold P Γs Γs' × P Γs Γs' d d'))


DEBUG:Going to execute:
match goal with
           | |- ?T => change (let _ := CoreTactics.block in T)
           end