Optional extensions

If everything is working, here are some additional things you might consider working on:

• Add more tests.
• Add support for unary negation. This documentation will be helpful.
• Add support for let expressions (!!).
• Improve some of the existing features, such as providing better error messages.
• Add support for generating a graphviz picture of the AST or for text-based trees.
• Add a simple optimizer for expressions, and allow the user to enable the optimizer with a flag. The optimizer would traverse the AST and simplify expressions that:
  • multiply by 1
  • multiply by 0
  • add 0
  • subtract 0
  • divide by 1
• Anything else that comes to mind!