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) [] ∥
  x : A
  ============================
   (Some x = None
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in
      ∃ D : ∀ y : C, Q y → P y x,
        nth_error (map_All fn [] Hl) 0 = Some (fn x D))


DEBUG:Going to execute:
simplify * || simplify ?; cbv beta