Goal:
  
  A : Type
  B : Type
  f : A → B
  g : A → B
  a : A
  l : list A
  IHl : map f l = map g l → All (λ x : A, f x = g x) l
  H0 : f a = g a
  H1 : map f l = map g l
  ============================
   (All (λ x : A, f x = g x) (a :: l))


DEBUG:Going to execute:
constructor