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 : forallb p l = true → Forall P l
  ============================
   (forallb p (x :: l) = true → Forall P (x :: l))


DEBUG:Going to execute:
<coq-core.plugins.ssreflect::ssrrewrite@0> $1 $2