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
  Hl : ∀ y : C, Q y → ∥ All (P y) [] ∥
  l : list A
  H : ∀ y : C, Q y → ∥ All (P y) l ∥
  n : nat
  x : A
  ============================
   (nth_error l n = Some x
    → {| pr1 := []; pr2 := Hl |} = {| pr1 := l; pr2 := H |}
      → [] = map_All fn l 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:
simplify * || simplify ?