public BudgetCategory AddTransaction(FinancialTransaction transaction)
{
if (!IsClosed) {
FinancialTransactions newTransactions = new FinancialTransactions();
newTransactions.AddRange(this.transactions);
newTransactions.Add(transaction);
return new BudgetCategory(this.Name, this.Budget, this.IsClosed, newTransactions);
} else {
throw new BudgetIsClosedException();
}
}