Goal:
  
  A : Type
  B : Type
  f : A → B
  a : A
  l : list A
  h : ∀ x y : A, f x = f y → x = y
  e : map f (a :: l) = map f []
  IHl : ∀ l' : list A, map f l = map f l' → l = l'
  ============================
   (a :: l = [])


DEBUG:Going to execute:
discriminate