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
  a0 : nth_error Γ0 n = Some x
  a1 : All2_fold P (skipn (S n) Γ0) (skipn (S n) Γ')
  b0 : P (skipn (S n) Γ0) (skipn (S n) Γ') x 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@{p0:=(a0, (a1, b0))} d')


DEBUG:Going to execute:
assert X2; [ exact (id0 id1) | clear id0 ]