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:
solve
[ do
![ solve
 [ trivial | apply : eq_sym ; trivial ]
 | discriminate
 | contradiction
 | Ltac.split ]
| case not_locked_false_eq_true; Ltac.assumption
| match goal with
  | H:¬ _ |- _ => solve [ case H; trivial ]
  end ]