In order to run this program, you need use command 'alias python=python3' to switch to python3

and use command 'python testDriver.py <input.txt >output.txt' to run the program with test script.

or use command 'python interpreter.py' to run the program with shell version.

''' Extra defined functions

1. Let
2. multiply
3. equal
4. cons
5. length
6. reverse
7. or
8. and '''