Goal:
  
  A : Type
  P : A → Prop
  Q : A → Prop
  p : A → bool
  Hp : ∀ x : A, Q x → reflect (P x) (p x)
  HP : Forall P []
  HP0 := {| pr1 := []; pr2 := HP |}
      : sigma (λ index : list A, Forall P index)
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in forallb p [] = true)


DEBUG:Going to execute:
hnf in (value of id')