Goal:
  
  X : Type
  Y : Type
  f : ∀ x : X, In x [] → Y
  g : ∀ x : X, In x [] → Y
  all_eq : ∀ (x : X) (isin : In x []), f x isin = g x isin
  ============================
   (map_In [] f = map_In [] g)


DEBUG:Going to execute:
easy