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 := A;
                pr2 := {| pr1 := B; pr2 := {| pr1 := l; pr2 := g |} |}
              |}
           : sigma
               (λ A : Type,
                  sigma
                    (λ B : Type, sigma (λ l : list A, ∀ x : A, In x l → B)))
  packcall_fn := {|
                   pr1 := packcall;
                   pr2 :=
                     map_In (pr1 (pr2 (pr2 packcall)))
                       (pr2 (pr2 (pr2 packcall)))
                 |}
              : sigma
                  (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)))
  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