public void Fire(Valet valet)
        {
            if (!valets.Contains(valet))
            {
                throw new ValetNotHiredException();
            }

            valets.Remove(valet);
        }
        public void Hire(Valet valet)
        {
            if (valets.Contains(valet))
            {
                throw new ValetAlreadyHiredException();
            }

            valets.Add(valet);
        }