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
  H0 : None = Some x
  H1 := {| pr1 := Some x; pr2 := H0 |}
     : sigma (λ index : option A, None = index)
  H2 := Some x : option A
  H3 := H0 : (λ index : option A, None = index) 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:
generalize (eq_refl : t = id'1); clearbody id'2 id'1; clear id' id;
            try unfold signature in id'2; hnf in id'2; 
            simpl in id'2