Goal:
  
  A : Type
  B : Type
  n : nat
  f : nat → A → B
  k : nat
  a : A
  l : list A
  IHn : ∀ (f : nat → A → B) (k : nat) (l : list A),
          skipn n (mapi_rec f l k) = mapi_rec f (skipn n l) (n + k)
  ============================
   (mapi_rec f (skipn n l) (S (n + k)) = mapi_rec f (skipn n l) (S (n + k)))


DEBUG:Going to execute:
solve
[ trivial
with eq_true
| reflexivity
| symmetry; trivial
| contradiction
| congruence ]