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
  ============================
   (∀ (l : list A) (l0 : ∀ y : C, Q y → ∥ All (P y) l ∥) 
      (H := CoreTactics.block) (l1 : list A) (H0 : 
                                              ∀ y : C, 
                                                Q y → ∥ All (P y) l1 ∥) 
      (n : nat) (x : A),
      nth_error l1 n = Some x
      → {| pr1 := l; pr2 := l0 |} = {| pr1 := l1; pr2 := H0 |}
        → map_All fn l l0 = map_All fn l1 H0
          → let H1 := CoreTactics.block in
            ∃ D : ∀ y : C, Q y → P y x,
              nth_error (map_All fn l1 H0) n = Some (fn x D))


DEBUG:Going to execute:
unshelve refine_ho elimt; intros; cbv beta