Goal:
  
  A : Type
  P : A → Prop
  a : A
  l : list A
  l' : list A
  H0 : P a
  H1 : Forall P (l ++ l')
  H : Forall P l ∧ Forall P l'
  ============================
   (Forall P l → Forall P l' → Forall P (a :: l))


DEBUG:Going to execute:
intro