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 := {| pr1 := l; pr2 := H |}
           : sigma (λ l : list A, ∀ y : C, Q y → ∥ All (P y) l ∥)
  packcall_fn := {|
                   pr1 := packcall;
                   pr2 := map_All fn (pr1 packcall) (pr2 packcall)
                 |}
              : sigma
                  (λ _ : sigma (λ l : list A, ∀ y : C, Q y → ∥ All (P y) l ∥),
                     list B)
  ============================
   (packcall_fn =
    {| pr1 := packcall; pr2 := map_All fn (pr1 packcall) (pr2 packcall) |}
    → 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:
lazymatch goal with
           | |- ?x = ?y → ?P =>
                 let y' := eval unfold packcall in y in
                 change (x = y' → P)
           end