/// <summary>
        /// Provjerava da li je trenutni objekt u ispravnom stanju
        /// </summary>
        public virtual void IsValid()
        {
            var errors = new BusinessRulesException<Person>();

            if (!isOibValid()) {
                errors.AddErrorFor(x => x.Oib, "Oib is not valid.");
            }

            if (errors.Errors.Any()) {
                throw errors;
            }
        }
        public virtual void SetBill(Bill bill)
        {
            var errors = new BusinessRulesException<Maintenance>();

            if(bill.From.Oib != contractor.Oib) {
                errors.AddErrorFor(m => m.Bill,
                    "Constractor who issued bill isn't the same as contractor responsible for the maintenance.");
            }

            if(errors.Errors.Any()) {
                throw errors;
            }

            this.bill = bill;
        }