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


DEBUG:Going to execute:
rewrite skipn_S skipn_S