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 : (λ (l : list A) (Hl : ∀ y : C, Q y → ∥ All (P y) l ∥) (x : list B),
         ∀ (H := CoreTactics.block) (l0 : list A) 
           (Hl0 : ∀ y : C, Q y → ∥ All (P y) l0 ∥),
           {| pr1 := l; pr2 := Hl |} = {| pr1 := l0; pr2 := Hl0 |}
           → x = map_All fn l0 Hl0
             → let H0 := CoreTactics.block in #|map_All fn l0 Hl0| = #|l0|)
        xs (map_All_obligation_2 x xs h)
        (map_All fn xs (map_All_obligation_2 x xs h))
  ============================
   (let H := CoreTactics.block in
    #|map_All fn (x :: xs)
        (rew [λ l : list A, ∀ y : C, Q y → ∥ All (P y) l ∥] eq_refl in h)| =
    #|x :: xs|)


DEBUG:Going to execute:
simpl eq_rect