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
  H0 : nth_error [] 0 = Some x
  ============================
   (let H := 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:
fun id' =>
             match id' with
             | id => idtac
             | _ => revert id'; CoreTactics.revert_until id
             end