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 : sigma
               (λ A : Type,
                  sigma
                    (λ B : Type, sigma (λ l : list A, ∀ x : A, In x l → B)))
  ============================
   (∀ eqargs : packcall =
               {|
                 pr1 := A;
                 pr2 := {| pr1 := B; pr2 := {| pr1 := l; pr2 := g |} |}
               |},
      eq_rect packcall
        (λ projs : sigma
                     (λ A : Type,
                        sigma
                          (λ B : Type,
                             sigma (λ l : list A, ∀ x : A, In x l → B))),
           list (pr1 (pr2 projs)))
        (map_In (pr1 (pr2 (pr2 packcall))) (pr2 (pr2 (pr2 packcall))))
        {|
          pr1 := A; pr2 := {| pr1 := B; pr2 := {| pr1 := l; pr2 := g |} |}
        |} eqargs =
      map_In
        (pr1
           (pr2
              (pr2
                 {|
                   pr1 := A;
                   pr2 := {| pr1 := B; pr2 := {| pr1 := l; pr2 := g |} |}
                 |})))
        (pr2
           (pr2
              (pr2
                 {|
                   pr1 := A;
                   pr2 := {| pr1 := B; pr2 := {| pr1 := l; pr2 := g |} |}
                 |}))) → let H := CoreTactics.block in map_In l g = map f l)


DEBUG:Going to execute:
match goal with
           | packcall:?type
             |- _ => change (let _ := c in type) in (type of packcall)
           end