Goal:
  
  A : Type
  P : A → Type
  a : A
  l : list A
  h : ∀ (i : nat) (x : A), nth_error (a :: l) i = Some x → P x
  IHl : (∀ (i : nat) (x : A), nth_error l i = Some x → P x) → All P l
  i : nat
  x : A
  e : nth_error l i = Some x
  ============================
   (nth_error (a :: l) (S i) = Some x)


DEBUG:Going to execute:
assumption
DEBUG LTAC Evaluated term: IHl
DEBUG LTAC Evaluated term: (h (S i))