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) [] ∥
  n : nat
  x : A
  H2 : option A
  H0 : None = H2
  ============================
   (Some x = H2
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in
      ∃ D : ∀ y : C, Q y → P y x,
        nth_error (map_All fn [] Hl) (S n) = Some (fn x D))


DEBUG:Going to execute:
try rename id into id',id' into id
Level 3: evaluation returns
try rename id into id',id' into id; cont id'1 id
where
t := Some x
of type
constr
id'1 := H2
of type
constr
id' := H0
of type
constr
id := H0
of type
constr
cont := fun id id' => DepElim.decompose_exists id id'
        where
        id := H0
        of type
        constr
of type
tacvalue of type tacvalue