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
  ============================
   (locked false = true)


DEBUG:Going to execute:
fun => <coq-core.plugins.ltac::assumption@0>