Goal:
  
  map_In : ∀ (A B : Type) (l : list A), (∀ x : A, In x l → B) → list B
  A : Type
  B : Type
  x0 : A
  xs : list A
  f : ∀ x : A, In x (x0 :: xs) → B
  x : A
  H : In x xs
  ============================
   (x0 = x ∨ In x xs)


DEBUG:Going to execute:
match H with
           | H' => fail 2
           | _ => unify X Y; clear H' || clear H
           end