Goal:
  
  A : Type
  P : A → Prop
  Q : A → Prop
  l : list A
  Hl : Forall P l
  Hq : Forall Q l
  ============================
   (Forall (λ x : A, P x ∧ Q x) l)


DEBUG:Going to execute:
induction Hl; inv Hq; constructor