Goal:
  
  A : Type
  B : Type
  f : A → B
  l : list A
  g : ∀ x : A, In x l → B
  Heqg : g = λ (x : A) (_ : In x l), f x
  ============================
   (map_In l g = map f l)


DEBUG:Going to execute:
constr:(fun_elim)
Level 6: Backtrace:


Could not find an instance for "FunctionalElimination 
                                  (map_In l g) ?fun_elim_ty 
                                  ?n"
in
environment:
A : Type
B : Type
f : A → B
l : list A
g : ∀ x : A, In x l → B
Heqg : g = λ (x : A) (_ : In x l), f x