Goal:
  
  A : Type
  B : Type
  l : list A
  f : ∀ x : A, In x l → B
  g : A → B
  hin : ∀ (x : A) (hin : In x l), f x hin = g x
  ============================
   (∀ (x : A) (isin : In x l), f x isin = g x)


DEBUG:Going to execute:
trivial with eq_true