$39.$ Implement the Function exchange

The implementation of exchange will require many $($but not all$)$ of the functions that you have implemented so far as helpers. Think about the functions you have implemented as $$building blocks$$ and how they can help you implement this function.

As always use the test script testcurrency.py to aid your development before checking your answer below.

Warning: The verification process below is potentially slow, since it has to connect to the web service.