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)
  HP1 := [] : list A
  HP2 := HP : (λ index : list A, Forall P index) HP1
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in forallb p [] = true)


DEBUG:Going to execute:
generalize (eq_refl : t = id'1); clearbody id'2 id'1; clear id' id;
            try unfold signature in id'2; hnf in id'2