Goal:
  
  A : Type
  P : A → Prop
  x : A
  l : list A
  H : P x
  h1 : Forall P l
  IHh1 : ∀ l2 : list A, Forall P l2 → Forall P (l ++ l2)
  l2 : list A
  h2 : Forall P l2
  ============================
   (Forall P (x :: l ++ l2))


DEBUG:Going to execute:
constructor