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)))
  ============================
   (∀ (H := CoreTactics.block) (A 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 A0 : Type,
                            sigma
                              (λ B0 : Type,
                                 sigma
                                   (λ l0 : list A0, ∀ x : A0, In x l0 → B0))),
               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 H0 := CoreTactics.block in map_In l g = map f l)


DEBUG:Going to execute:
cbv zeta in packcall
DEBUG LTAC Evaluated term: (∀ (H := CoreTactics.block) 
                              (A 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 A0 : Type,
                                                 sigma
                                                 (λ B0 : Type,
                                                 sigma
                                                 (λ l0 : list A0,
                                                 ∀ x : A0, In x l0 → B0))),
                                       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 H0 := CoreTactics.block in
                                    map_In l g = map f l)