"Minus" not supported, but README.md says that it is

[catherinedevlin]

Python 3.7.1 (default, Oct 25 2018, 14:53:35) 
[GCC 8.2.1 20181011 (Red Hat 8.2.1-4)] on linux

>>> from word_calculator import calc
>>> calc('10 minus 5')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/catherine/org/ddl/foss-contribution-exercise/word_calculator.py", line 9, in calc
    operator_func = FUNCTIONS[operator]
KeyError: 'minus'