Goal:
  
  X : Type
  Y : Type
  a : X
  xs : list X
  f : ∀ x : X, In x (a :: xs) → Y
  g : ∀ x : X, In x (a :: xs) → Y
  IHxs : ∀ f g : ∀ x : X, In x xs → Y,
           (∀ (x : X) (isin : In x xs), f x isin = g x isin)
           → map_In xs f = map_In xs g
  all_eq : ∀ (x : X) (isin : In x (a :: xs)), f x isin = g x isin
  ============================
   (map_In (a :: xs) f = map_In (a :: xs) g)


DEBUG:Going to execute:
idtac