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
  l : list A
  H : ∀ y : C, Q y → ∥ All (P y) l ∥
  n : nat
  x : A
  H0 : nth_error l n = Some x
  packcall : let H := map_All fn in
             sigma (λ l : list A, ∀ y : C, Q y → ∥ All (P y) l ∥)
  ============================
   (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:
match goal with
           | H:_ |- _ => is_secvar H; tac H
           end