Goal:
  
  A : Type
  B : Type
  C : Type
  Q : C → Type
  P : C → A → Prop
  fn : ∀ x : A, (∀ y : C, Q y → P y x) → B
  x : A
  xs : list A
  h : ∀ y : C, Q y → ∥ All (P y) (x :: xs) ∥
  H : ∀ (n : nat) (x0 : A),
        nth_error xs n = Some x0
        → ∃ D : ∀ y : C, Q y → P y x0,
            nth_error (map_All fn xs (map_All_obligation_2 x xs h)) n =
            Some (fn x0 D)
  n : nat
  x0 : A
  H0 : Some x0 = nth_error xs n
  ============================
   (nth_error xs n = Some x0)


DEBUG:Going to execute:
symmetry; trivial
Level 1: Backtrace:

In environment
A : Type
B : Type
C : Type
Q : C → Type
P : C → A → Prop
fn : ∀ x : A, (∀ y : C, Q y → P y x) → B
x : A
xs : list A
h : ∀ y : C, Q y → ∥ All (P y) (x :: xs) ∥
H : ∀ (n : nat) (x0 : A),
      nth_error xs n = Some x0
      → ∃ D : ∀ y : C, Q y → P y x0,
          nth_error (map_All fn xs (map_All_obligation_2 x xs h)) n =
          Some (fn x0 D)
n : nat
x0 : A
H0 : Some x0 = nth_error xs n
Unable to unify "Some x0" with "nth_error xs n".
Level 0: Backtrace:

In environment
A : Type
B : Type
C : Type
Q : C → Type
P : C → A → Prop
fn : ∀ x : A, (∀ y : C, Q y → P y x) → B
x : A
xs : list A
h : ∀ y : C, Q y → ∥ All (P y) (x :: xs) ∥
H : ∀ (n : nat) (x0 : A),
      nth_error xs n = Some x0
      → ∃ D : ∀ y : C, Q y → P y x0,
          nth_error (map_All fn xs (map_All_obligation_2 x xs h)) n =
          Some (fn x0 D)
n : nat
x0 : A
H0 : Some x0 = nth_error xs n
Unable to unify "Some x0" with "nth_error xs n".