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 : ∀ (H := CoreTactics.block) (l : list A) (H0 : 
                                               ∀ y : C, 
                                                 Q y → ∥ All (P y) l ∥) 
        (n : nat) (x0 : A),
        nth_error l n = Some x0
        → {| pr1 := xs; pr2 := map_All_obligation_2 x xs h |} =
          {| pr1 := l; pr2 := H0 |}
          → map_All fn xs (map_All_obligation_2 x xs h) = map_All fn l H0
            → let H1 := CoreTactics.block in
              ∃ D : ∀ y : C, Q y → P y x0,
                nth_error (map_All fn l H0) n = Some (fn x0 D)
  n : nat
  x0 : A
  H1 : nth_error (x :: xs) n = Some x0
  ============================
   (let H := CoreTactics.block in
    ∃ D : ∀ y : C, Q y → P y x0,
      nth_error (map_All fn (x :: xs) h) n = Some (fn x0 D))


DEBUG:Going to execute:
cbn beta in hyp; eqns_specialize_eqs_block hyp;
            cbn[eq_rect_r eq_rect] in hyp