Goal:
  
  packcall : let H := @map_In in
             sigma
               (λ A : Type,
                  sigma
                    (λ B : Type, sigma (λ l : list A, ∀ x : A, In x l → B)))
  A : Type
  B : Type
  f : A → B
  l : list A
  ============================
   (∀ g : ∀ x : A, In x l → B,
      g = (λ (x : A) (_ : In x l), f x)
      → ∀ 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:
fun tac => lazymatch goal with
                      | H:_ |- _ => tac H
                      end