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 : sigma (λ l : list A, ∀ y : C, Q y → ∥ All (P y) l ∥)
  ============================
   (∀ (H := CoreTactics.block) (l : list A) (H0 : 
                                             ∀ y : C, 
                                               Q y → ∥ All (P y) l ∥) 
      (n : nat) (x : A),
      nth_error l n = Some x
      → packcall = {| pr1 := l; pr2 := H0 |}
        → map_All fn (pr1 packcall) (pr2 packcall) =
          map_All fn (pr1 {| pr1 := l; pr2 := H0 |})
            (pr2 {| pr1 := l; pr2 := H0 |})
          → let H1 := CoreTactics.block in
            ∃ D : ∀ y : C, Q y → P y x,
              nth_error (map_All fn l H0) n = Some (fn x D))


DEBUG:Going to execute:
<coq-core.plugins.ltac::revert@0> $1