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:
set (call := p);
            lazymatch goal with
            | call:=?fp:_ |- _ => subst call; Init.funelim_constr fp
            end