Goal:
  
  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'
  ============================
   (∑ d : A,
      nth_error Γ (S n) = Some d
      × (let Γs := skipn (S (S n)) Γ in
         let Γs' := skipn (S (S n)) [] in All2_fold P Γs Γs' × P Γs Γs' d d'))


DEBUG:Going to execute:
change (let _ := CoreTactics.block in T)
DEBUG LTAC Evaluated term: (let H := CoreTactics.block in
                            ∑ d : A,
                              nth_error Γ 0 = Some d
                              × (let Γs := skipn 1 Γ in
                                 let Γs' := skipn 1 (a :: Γ') in
                                 All2_fold P Γs Γs' × P Γs Γs' d d'))