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 ∥)
  ============================
   (∀ (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:
cbv zeta in packcall
DEBUG LTAC Evaluated term: (∀ (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))