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) (Hl : ∀ y : C, Q y → ∥ All (P y) l ∥),
      packcall = {| pr1 := l; pr2 := Hl |}
      → map_All fn (pr1 packcall) (pr2 packcall) =
        map_All fn (pr1 {| pr1 := l; pr2 := Hl |})
          (pr2 {| pr1 := l; pr2 := Hl |})
        → let H := CoreTactics.block in #|map_All fn l Hl| = #|l|)


DEBUG:Going to execute:
fun tac => lazymatch goal with
                      | H:_ |- _ => tac H
                      end