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


DEBUG:Going to execute:
destruct n => /=