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
  x : A
  xs : list A
  h : ∀ y : C, Q y → ∥ All (P y) (x :: xs) ∥
  H : ∀ (n : nat) (x0 : A),
        nth_error xs n = Some x0
        → ∃ D : ∀ y : C, Q y → P y x0,
            nth_error (map_All fn xs (map_All_obligation_2 x xs h)) n =
            Some (fn x0 D)
  x0 : A
  H1 : Some x = Some x0
  H0 := {| pr1 := Some x0; pr2 := H1 |}
     : sigma (λ index : option A, Some x = index)
  H2 := Some x0 : option A
  H3 := H1 : (λ index : option A, Some x = index) H2
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in
    ∃ D : ∀ y : C, Q y → P y x0,
      nth_error (map_All fn (x :: xs) h) 0 = Some (fn x0 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; rename id'2 into id; cont id id id'1 t