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
  packcall : let H := map_All fn in
             sigma (λ l : list A, ∀ y : C, Q y → ∥ All (P y) l ∥)
  l : list A
  H : ∀ y : C, Q y → ∥ All (P y) l ∥
  n : nat
  x : A
  ============================
   (nth_error l n = Some x
    → packcall = {| pr1 := l; pr2 := H |}
      → map_All fn (pr1 packcall) (pr2 packcall) =
        map_All fn (pr1 {| pr1 := l; pr2 := H |})
          (pr2 {| pr1 := l; pr2 := H |})
        → let H0 := CoreTactics.block in
          ∃ D : ∀ y : C, Q y → P y x,
            nth_error (map_All fn l H) n = Some (fn x D))


DEBUG:Going to execute:
tac H