Goal:
  
  A : Type
  P : A → Prop
  Q : A → Prop
  p : A → bool
  x : A
  l : list A
  H : Q x
  HQ : Forall Q l
  Hp : ∀ x : A, Q x → reflect (P x) (p x)
  IHHQ : Forall P l → forallb p l = true
  HP : Forall P (x :: l)
  HP0 := {| pr1 := x :: l; pr2 := HP |}
      : sigma (λ index : list A, Forall P index)
  HP1 := x :: l : list A
  HP2 := HP : (λ index : list A, Forall P index) HP1
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in forallb p (x :: l) = 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