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:
match goal with
           | |- ?T => change (let _ := CoreTactics.block in T)
           end