Your test suite from the previous exercise should continue to work with your new interpreter. However, it needs to be extended.
If you are working on a single-binding, single-argument function interpreter, then you need tests that stress your with statements and functions.
If you are working on a multi-binding, multi-argument function interpreter, then you need to test multiple bindings and multiple arguments as well.
Strategy
Write test cases before you extend your interpreter. You should know what kind of code your parser will handle, and what values will come back from your interpreter.
When you write a test case, send it to the class mailing list. Flag the test as either "SBSA: " or "MBMA: " so people know what stage of the lab you are working on. Therefore, everyone will benefit from your efforts, and visa-versa.