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) (Hl : 
                                              ∀ y : C, 
                                                Q y → ∥ All (P y) l1 ∥),
      {| pr1 := l; pr2 := l0 |} = {| pr1 := l1; pr2 := Hl |}
      → map_All fn l l0 = map_All fn l1 Hl
        → let H0 := CoreTactics.block in #|map_All fn l1 Hl| = #|l1|)


DEBUG:Going to execute:
unshelve refine_ho elimt; intros; cbv beta;
            DepElim.simplify_dep_elim; CoreTactics.intros_until_block;
            DepElim.simplify_dep_elim;
            cbn beta iota
             delta [Prop.Logic.transport Logic.eq_elim Logic.eq_elim_r
                   eq_rect DepElim.pack_sigma_eq
                   DepElim.pack_sigma_eq_nondep] in *;
            DepElim.simplify_IH_hyps