Goal:
  
  A : Type
  B : Type
  packcall : let H := map_InP in sigma (λ l : list A, ∀ x : A, In x l → B)
  ============================
   (∀ (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 H := CoreTactics.block in map_InP l g = map f l)


DEBUG:Going to execute:
fun tac => lazymatch goal with
                      | H:_ |- _ => tac H
                      end