Goal:
  
  A : Type
  f : A → bool
  l : list A
  g : ∀ x : A, In x l → bool
  Heqg : g = λ (x : A) (_ : In x l), f x
  packcall : sigma (λ l : list A, ∀ x : A, In x l → bool)
  ============================
   (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 packcall c =>
             match goal with
             | packcall:?type
               |- _ => change (let _ := c in type) in (type of packcall)
             end