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
                × (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 Γ0 Γ'
  p : P Γ0 Γ' d a
  ============================
   (let H := CoreTactics.block in
    nth_error (a :: Γ') (S n) = Some d'
    → let H0 := CoreTactics.block in
      ∑ d0 : A,
        nth_error (d :: Γ0) (S n) = Some d0
        × (let Γs := skipn (S (S n)) (d :: Γ0) in
           let Γs' := skipn (S (S n)) (a :: Γ') in
           All2_fold P Γs Γs' × P Γs Γs' d0 d'))


DEBUG:Going to execute:
PTRDEBUGTACmatch goal with
           | |- let _ := CoreTactics.block in ?T =>
                 match T with
                 | context [ CoreTactics.block ] =>
                     change T; CoreTactics.intros_until_block
                 end
           | _ => CoreTactics.unblock_goal
           end