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
  packcall := {| pr1 := l; pr2 := g |}
           : sigma (λ l : list A, ∀ x : A, In x l → B)
  packcall_fn := {|
                   pr1 := packcall;
                   pr2 := map_InP (pr1 packcall) (pr2 packcall)
                 |}
              : sigma
                  (λ _ : sigma (λ l : list A, ∀ x : A, In x l → B), list B)
  ============================
   (packcall_fn =
    {| pr1 := packcall; pr2 := map_InP (pr1 packcall) (pr2 packcall) |}
    → let H := CoreTactics.block in map_InP l g = map f l)


DEBUG:Going to execute:
lazymatch goal with
           | |- ?x = ?y → ?P =>
                 let y' := eval unfold packcall in y in
                 change (x = y' → P)
           end