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