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:
case not_locked_false_eq_true; Ltac.assumption
Level 1: Backtrace:

In environment
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
Unable to unify "option_map f (nth_error l (S (S n)))" with
 "nth_error (map f l) (S (S n))".
Level 0: Backtrace:

In environment
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
Unable to unify "option_map f (nth_error l (S (S n)))" with
 "nth_error (map f l) (S (S n))".
Level 0: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.