Goal:
  
  map_In : ∀ (H := fixproto) (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
  ============================
   (In x (x :: xs))


DEBUG:Going to execute:
simpl; intros