Goal:
  
  A : Type
  B : Type
  f : A → B
  l : nat
  H : ∀ l0 : list A, map f (skipn l l0) = skipn l (map f l0)
  l0 : list A
  ============================
   (map f (skipn (S l) l0) = skipn (S l) (map f l0))


DEBUG:Going to execute:
<coq-core.plugins.ltac::contradiction@0> $1
Level 0: Backtrace:

No primitive equality found.