Goal:
  
  a : Nbar.t
  l : list Nbar.t
  IHl : forall acc : Nbar.t,
        (Some 0 <= acc)%nbar -> (Some 0 <= fold_left max l acc)%nbar
  ============================
   (forall acc : Nbar.t,
    (Some 0 <= acc)%nbar -> (Some 0 <= fold_left max (a :: l) acc)%nbar)


DEBUG:Going to execute:
intros acc H