This PR changes the Currency-related types near the end of the book to something that probably makes more sense: just an abstract type HasMoney with relevant properties that can be extended. The former method was sort of interesting philosophically (that each person has various piles of money in currencies in banks, IOUs, salaries to be paid etc. that link to them) but feels overly complicated and weird in practice.
This PR changes the Currency-related types near the end of the book to something that probably makes more sense: just an abstract type HasMoney with relevant properties that can be extended. The former method was sort of interesting philosophically (that each person has various piles of money in currencies in banks, IOUs, salaries to be paid etc. that link to them) but feels overly complicated and weird in practice.