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 forallb p [] = true)


DEBUG:Going to execute:
match id' with
           | id => idtac
           | _ => revert id'; CoreTactics.revert_until id
           end