Extending the Charme Interpreter by Defining Null and Null?

I need to help extending the Charme interpreter and de开发者_高级运维fining the null and null? primitives that behave similarly to the primitive Scheme procedures. I know I can use Python's None value to represent null.

This seems to be a homework question: http://www.cs.virginia.edu/~evans/cs1120-f09/ps/ps7/

A few pointers:

• How is the empty list (aka null) represented as a Python value?

• Which function is used to extend the environment?