Goal:
  
  X : Type
  Y : Type
  xs : list X
  f : ∀ x : X, In x xs → Y
  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)


DEBUG:Going to execute:
induction xs in xs, f, g |- *; intros all_eq
DEBUG LTAC Evaluated term: IHxs