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


DEBUG:Going to execute:
assumption
DEBUG LTAC Evaluated term: (h 0 a eq_refl)