Goal:
  
  A : Type
  p : A → bool
  a : A
  l : list A
  IHl : forallb p l → Forall (λ x : A, p x) l
  ============================
   (p a && forallb p l → Forall (λ x : A, p x) (a :: l))


DEBUG:Going to execute:
trivial; hnf