Goal:
  
  A : Type
  B : Type
  f : A → B
  g : A → B
  x : A
  l : list A
  H : f x = g x
  H0 : Forall (λ x : A, f x = g x) l
  IHForall : map f l = map g l
  ============================
   (g x :: map g l = g x :: map g l)


DEBUG:Going to execute:
trivial with eq_true