Goal:
  
  A : Type
  B : Type
  packcall : let H := map_InP in sigma (λ l : list A, ∀ x : A, In x l → B)
  ============================
   (∀ (H := CoreTactics.block) (f : A → B) (l : list A) 
      (g : ∀ x : A, In x l → B),
      g = (λ (x : A) (_ : In x l), f x)
      → packcall = {| pr1 := l; pr2 := g |}
        → map_InP (pr1 packcall) (pr2 packcall) =
          map_InP (pr1 {| pr1 := l; pr2 := g |})
            (pr2 {| pr1 := l; pr2 := g |})
          → let H0 := CoreTactics.block in map_InP l g = map f l)


DEBUG:Going to execute:
cbv zeta in packcall
DEBUG LTAC Evaluated term: (∀ (H := CoreTactics.block) 
                              (f : A → B) (l : list A) 
                              (g : ∀ x : A, In x l → B),
                              g = (λ (x : A) (_ : In x l), f x)
                              → packcall = {| pr1 := l; pr2 := g |}
                                → map_InP (pr1 packcall) (pr2 packcall) =
                                  map_InP (pr1 {| pr1 := l; pr2 := g |})
                                    (pr2 {| pr1 := l; pr2 := g |})
                                  → let H0 := CoreTactics.block in
                                    map_InP l g = map f l)