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
  ============================
   (∀ Hl0 : ∀ y : C, Q y → ∥ All (P y) l ∥,
      {| pr1 := []; pr2 := Hl |} = {| pr1 := l; pr2 := Hl0 |}
      → [] = map_All fn l Hl0
        → let H := CoreTactics.block in #|map_All fn l Hl0| = #|l|)


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