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 : let l := xs in
      let Hl : ∀ y : C, Q y → ∥ All (P y) l ∥ := map_All_obligation_2 x xs h
        in
      #|map_All fn l Hl| = #|l|
  ============================
   (let H := CoreTactics.block in #|map_All fn (x :: xs) h| = #|x :: xs|)


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