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)))
  ============================
   (∀ e : 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 |} |}
        |} e =
      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:
intros H Heq; try (rewrite <- Heq; clear Heq);
            CoreTactics.revert_until H