Goal:
  
  A : Type
  P : A → Prop
  x : A
  x0 : A
  l : list A
  H : P x0
  HPl : Forall P l
  IHHPl : ∀ n : nat, nth_error l n = Some x → P x
  ============================
   (nth_error (x0 :: l) 0 = Some x → P x)


DEBUG:Going to execute:
t; easy