public Transaction(Money amount, TransactionType transactionType)
            : this()
        {
            if (transactionType == TransactionType.Unknown)
            {
                throw new ArgumentException("Please specify a debit or credit transaction.");
            }

            if (amount == Money.Zero)
            {
                throw new ArgumentException("A transaction cannot have a zero value.");
            }

            this.transactionType = transactionType;
            this.amount = amount;
        }
 public void Debit(Money amount)
 {
     transactions.Add(new Transaction(amount, TransactionType.Debit));
 }