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


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