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
  call := map_In l g : list B
  ============================
   (call = map f l)


DEBUG:Going to execute:
subst call