практикум 15_3

 class Program
    {
        class BD
        {
            class Abonent
            {
                public string name;
                public string phone;
                public string tariff;
                public Abonent left;
                public Abonent right;

                public Abonent(string name_, string phone_, string tariff_)
                {
                    name = name_;
                    phone = phone_;
                    tariff = tariff_;
                    left = null;
                    right = null;
                }
            }

            public BD
        }