Goal:
  
  A : Type
  B : Type
  x : A
  xs : list A
  f0 : A → B
  H : ∀ (A0 := A) (B := B) (f : A0 → B),
        let l := xs in
        let g := λ (x : A) (_ : In x xs), f0 x in
        g = (λ (x : A0) (_ : In x l), f x) → map_In l g = map f l
  ============================
   (let H := CoreTactics.block in
    map_In (x :: xs) (λ (x0 : A) (_ : In x0 (x :: xs)), f0 x0) =
    map f0 (x :: xs))


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