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:
match c with
           | context [ ?f ] => constr:(fun_elim)
           end