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
  ============================
   (All P (a :: l))


DEBUG:Going to execute:
<coq-core.plugins.ltac::constructor@0>