Goal:
  
  A : Type
  a : A
  l : list A
  n : nat
  IHl : ∀ n : nat, firstn n (firstn (S n) l) = firstn n l
  ============================
   (a :: firstn n (firstn (S n) l) = a :: firstn n l)


DEBUG:Going to execute:
rewrite IHl