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:
Ltac.assumption
DEBUG LTAC Evaluated term: not_locked_false_eq_true