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
  p0 : nth_error Γ'0 n = Some x
       × All2_fold P (skipn (S n) Γ) (skipn (S n) Γ'0)
         × P (skipn (S n) Γ) (skipn (S n) Γ'0) d x
  ============================
   (∀ (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:
do 2 intro
DEBUG LTAC Evaluated term: p0