Goal:
  
  A : Type
  f : A → bool
  l : list A
  g : ∀ x : A, In x l → bool
  Heqg : g = λ (x : A) (_ : In x l), f x
  ============================
   (forallb_InP l g = forallb f l)


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


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