Goal:
  
  A : Type
  B : Type
  f : A → B
  n : nat
  H : ∀ l : list A,
        nth_error (map f l) (S n) = option_map f (nth_error l (S n))
  l : list A
  ============================
   (nth_error (map f l) (S (S n)) = option_map f (nth_error l (S (S n))))


DEBUG:Going to execute:
do
![ solve
 [ trivial | apply : eq_sym ; trivial ]
 | discriminate
 | contradiction
 | Ltac.split ]