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:
<coq-core.plugins.ltac::trivial@0> $1 $2

File "./theories/MCList.v", line 663, characters 15-32:
Warning: Notation List.skipn_length is deprecated since 8.20.
Use length_skipn instead.
[deprecated-syntactic-definition-since-8.20,deprecated-since-8.20,deprecated-syntactic-definition,deprecated,default]