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 []
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in forallb p [] = true)


DEBUG:Going to execute:
fun id =>
             DepElim.generalize_sig id
              ltac:(fun id id' => DepElim.decompose_exists id id')