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


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