text: Title: Unit tests for reference.py
  Body: - Need unit tests on a few functions in `testbook/reference.py`.
  
  <img width="609" alt="Screenshot 2021-05-22 at 10 25 18 PM" src="https://user-images.githubusercontent.com/39169550/119234637-982e1e80-bb4c-11eb-8c33-eb463a37b8ff.png">
  
  Command to generate above output:
  
  ```bash
  $ cd testbook
  $ pytest --cov=testbook --cov-report=html
  $ open htmlcov/index.html # or just navigate to this directory using file explorer (or finder) and open index.html
  ```