test_extend_free_period (__main__.ExtendAcceptance.test_extend_free_period) ... ERROR

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ERROR: test_extend_free_period (__main__.ExtendAcceptance.test_extend_free_period)
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Traceback (most recent call last):
  File "/Users/magicliang/Desktop/Programming/git/hexo-blog/examples/software-modeling/labs/18/acceptance.py", line 11, in test_extend_free_period
    actual = book.extend("R1", 5)
  File "/Users/magicliang/Desktop/Programming/git/hexo-blog/examples/software-modeling/labs/18/baseline.py", line 51, in extend
    raise NotImplementedError("F02 not implemented")
NotImplementedError: F02 not implemented

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Ran 1 test in 0.003s

FAILED (errors=1)
