text: I'm writing a piece of software for simulating double-entry book-keeping. My software has an "account" class for assets, liabilities, equity, revenues, and expenses. It also has a "transaction" class, that can debit and credit these accounts. How should I track cash-flow expenses in my software? Should I add cash flow type (operations, investing, and financing) as properties to my "account" class or to my "transaction" class?