docs: def equation(operation, firstnum, secondnum):
      """
      Solve a simple maths equation manually
      """
      if operation == 'plus':
          return firstnum + secondnum
      elif operation == 'minus':
          return firstnum - secondnum
      elif operation == 'multiply':
          return firstnum * secondnum
      elif operation == 'divide':
          if not secondnum == 0:
              return firstnum / secondnum
          raise ZeroDivisionError("Unable to divide by 0.")
      raise ValueError('Invalid operation provided.')