In F#, implement an interpreter for a simple arithmetic language.

The language has to include:
- numbers
- a few arithmetic operations

That's it!

You must specify test cases.  If you don't write test cases, I will assume that
your code is incorrect.

For this lab, you're not required to build any kind of parser.  That is, you
may define your abstract syntax trees any way you like, and specify your test
cases by passing ASTs to the interpreter.