Goal:
  
  A : Type
  m : nat
  x : A
  IHm : ∀ (n : nat) (l : list A),
          nth_error (firstn n l) m = Some x → nth_error l m = Some x
  a : A
  l : list A
  H : None = Some x
  ============================
   (nth_error l m = Some x)


DEBUG:Going to execute:
inversion H