nltk/nltk — pull request #237 by slattarini (2012-02-29 22:30:15+00:00)
Title: prefer NotImplementedError over AssertionError in few places

The python documentation on "Built-in Exceptions":

reads:

``In user defined base classes, abstract methods should raise
this exception when they require derived classes to override
the method.''

But existing NLTK code didn't consistently follow that best practice,
sometimes using 'AssertionError' or 'assert' in similar situations.
Fix that. Also, make error messages less verbose, for consistency
with other existing code.