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
  ============================
   (∃ D : ∀ y : C, Q y → P y x,
      nth_error (map_All fn [] Hl) 0 = Some (fn x D))


DEBUG:Going to execute:
DepElim.do_intros H; DepElim.generalize_by_eqs H
Level 1: evaluation returns
DepElim.do_intros H; DepElim.generalize_by_eqs H; tac H
where
tac := fun hyp => DepElim.do_case hyp
       where
       id := H0
       of type
       constr
of type
tacvalue
H := H0
of type
constr of type tacvalue